PCShrinkv040b

Hunt Hypothesis

This rule detects the presence of PCShrink, a legacy compression utility often exploited by adversaries to compress and obfuscate malicious payloads or exfiltrated data before transfer. Proactively hunting for this indicator in Azure Sentinel helps identify potential data staging or obfuscation activities that may indicate an adversary preparing for exfiltration or hiding artifacts within the environment.

YARA Rule

rule PCShrinkv040b
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 9C 60 BD [4] 01 [5] FF [5] 6A ?? FF [5] 50 50 2D }

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

• Legacy Application Deployment via Group Policy: When IT administrators deploy or update legacy line-of-business applications (e.g., older versions of Adobe Acrobat, Java Runtime, or specific banking clients) that rely on PCShrink for code compression, the executable or its installer may be scanned and flagged.
  • Filter/Exclusion: Exclude files located in specific application directories (e.g., C:\Program Files\LegacyApp\) or exclude the specific installer executable name (e.g., setup.exe or install_pcshrink.exe) if the hash is known and verified.
• Software Update Services (WSUS/SCCM) Delta Updates: Windows Server Update Services (WSUS) or System Center Configuration Manager (SCCM) often use delta update mechanisms that may involve compressed payloads or temporary staging files utilizing PCShrink technology during the update process on client machines.
  • Filter/Exclusion: Exclude processes or files originating from the Windows Update service (wuauclt.exe or usoclient.exe) or files located in the Windows Update cache directory (C:\Windows\SoftwareDistribution\).
• Antivirus/Endpoint Protection Quarantine Scans: When endpoint protection solutions (e.g., Microsoft Defender for Endpoint, CrowdStrike Falcon) perform deep scans or restore quarantined files, they may temporarily extract or process compressed archives that contain PCShrink-compressed binaries, triggering the YARA rule during the scan operation.
  • Filter/Exclusion: Exclude files with paths containing \Quarantine\, \Temp\, or \Cache\ associated with the specific EDR/AV vendor, or exclude events where the parent process is the EDR service executable (e.g., MsMpEng.exe for Defender).
• Third-Party Backup and Archiving Tools: Enterprise backup solutions (e.g., Veeam, Acronis, or Comm