PCShrinkerv020

Hunt Hypothesis

This rule detects the presence of the PCShrinker v0.20 malware, a known tool used for compressing and obfuscating payloads to evade static analysis. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify compromised endpoints where adversaries may be staging or executing compressed malicious code before it is deployed.

YARA Rule

rule PCShrinkerv020
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 E8 01 [2] 60 01 AD B3 27 40 ?? 68 }

condition:
		$a0 at pe.entry_point
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: A developer or build engineer uses PCShrinker (a legitimate .NET assembly optimizer) to reduce the size of a custom internal library or application before deploying it to the staging environment. The tool is executed from a standard development workstation or CI/CD agent, triggering the YARA rule on the executable or the resulting optimized assembly.

  • Filter/Exclusion: Exclude files located in specific development directories (e.g., C:\Dev\, C:\Builds\, or C:\CI\) or exclude processes spawned by known build agents (e.g., vstest.console.exe, dotnet.exe, or msbuild.exe) when the parent process is a recognized development tool.

• Scenario: An IT administrator performs a routine maintenance task on a legacy application server, using PCShrinker to optimize a large, outdated .NET application to improve startup times and reduce memory footprint. The tool is run manually via PowerShell or Command Prompt on a server in the AppServers OU.

  • Filter/Exclusion: Exclude detections on servers within the AppServers or LegacyApps Organizational Unit (OU) if the process is initiated by an administrative account (e.g., DOMAIN\svc-admin or DOMAIN\it-mgmt) and the file path is under a known application directory (e.g., C:\Apps\LegacyApp\).

• Scenario: A scheduled task or PowerShell script in a production environment uses PCShrinker as part of a nightly build pipeline to optimize dependencies before packaging a release artifact. The task runs under a service account (e.g., DOMAIN\svc-build) and writes the optimized assembly to a shared network drive or artifact repository.

  • Filter/Exclusion: Exclude detections where the parent process is schtasks.exe or powershell.exe and