Potential Script Proxy Execution Via CL_Mutexverifiers.ps1

Severity: medium
Author: Nasreddine Bencherchali (Nextron Systems), oscd.community, Natalia Shornikova, frack113
Source: upstream

Detects the use of the Microsoft signed script "CL_mutexverifiers" to proxy the execution of additional PowerShell script commands

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1216` System Script Proxy Execution

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation
Security-Auditing	4688	A new process has been created.

Stages and Predicates

Stage 1: `all of selection_pwsh`

or:
ParentImage|endswith: '\powershell.exe'
ParentImage|endswith: '\pwsh.exe'
CommandLine|contains: ' -nologo -windowstyle minimized -file '
Image|endswith: '\powershell.exe'

Stage 2: `all of selection_temp`

or:
CommandLine|contains: '\AppData\Local\Temp\'
CommandLine|contains: '\Windows\Temp\'

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`CommandLine`	match	`-nologo -windowstyle minimized -file` `\AppData\Local\Temp\` corpus 16 (sigma 16) `\Windows\Temp\` corpus 10 (sigma 10)
`Image`	ends_with	`\powershell.exe` corpus 143 (sigma 143)
`ParentImage`	ends_with	`\powershell.exe` corpus 16 (sigma 16) `\pwsh.exe` corpus 16 (sigma 16)