PCShrinkerv029

Hunt Hypothesis

This rule detects the presence of the PCShrinker malware, a tool often used by adversaries to compress and obfuscate payloads for stealthy execution or data exfiltration. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify compromised endpoints early, mitigating the risk of hidden malicious activity before it progresses to lateral movement or impact.

YARA Rule

rule PCShrinkerv029
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BD [4] 01 AD 55 39 40 ?? 8D B5 35 39 40 }

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 DevOps engineer uses a legitimate open-source compression utility (e.g., 7z, zip, or a custom Python script using zlib) that shares specific string constants or byte patterns with the “PCShrinker” malware family, causing the YARA rule to match the binary or script file.
  • Filter/Exclusion: Exclude files located in standard development directories (e.g., C:\dev\, C:\projects\, C:\src\) or specific known-good hashes for common compression tools. Alternatively, exclude processes where the parent process is a known IDE or build tool (e.g., code.exe, msbuild.exe, python.exe).
• Scenario: An IT administrator runs a scheduled maintenance job using a proprietary vendor tool (e.g., a disk cleanup utility or log rotation script) that internally uses a compression algorithm with a similar signature to PCShrinker, particularly if the tool is installed in a non-standard directory like C:\Program Files (x86)\VendorName\Maintenance\.
  • Filter/Exclusion: Exclude processes spawned by known vendor service executables (e.g., VendorService.exe) or files located in specific vendor installation paths. Add an exclusion for scheduled tasks that run known maintenance scripts with specific names (e.g., *cleanup*.bat, *rotate*.ps1).
• Scenario: A security team or SOC analyst performs a manual test or uses a sandboxed environment to analyze a sample of PCShrinker or a similar threat, and the YARA rule triggers on the test file or the analysis tool itself (e.g., yara.exe scanning a known PCShrinker sample in a dedicated test folder).
  • Filter/Exclusion: Exclude files and processes within dedicated security test directories (e.g., `C:\SOC\Testing\