CrunchPEv10xx

Hunt Hypothesis

This rule detects the presence of PE executables compiled with the CrunchPE packer, a technique often used by adversaries to compress malware binaries and evade static analysis tools. Proactively hunting for these packed files in Azure Sentinel helps identify potential low-severity threats or staging artifacts that may be hiding in endpoints or storage accounts before they are executed or expanded.

YARA Rule

rule CrunchPEv10xx
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 E8 [4] 5D 83 ED 06 8B C5 55 60 89 AD [4] 2B 85 [4] 89 85 [4] 80 BD [5] 75 09 C6 85 }

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: Execution of legacy or custom internal build tools that utilize the Crunch PE packer (e.g., cruncher.exe or crunchpe.exe) during CI/CD pipeline artifact packaging or nightly build jobs.
  • Filter/Exclusion: Exclude processes running from specific build agent directories (e.g., C:\Jenkins\workspace\, C:\AzureDevOps\agent\_work\) or exclude parent processes like msbuild.exe, dotnet.exe, or java.exe when the child process name matches crunch*.exe.
• Scenario: Deployment of third-party software installers or updaters that bundle executables packed with Crunch PE to reduce file size (e.g., certain versions of 7-Zip, WinRAR, or niche vendor agents like F5 BigIP or Citrix Receiver).
  • Filter/Exclusion: Allowlist known vendor installers by their specific file paths (e.g., C:\Program Files\7-Zip\Uninstall.exe, C:\Program Files (x86)\WinRAR\Uninstall.exe) or exclude processes spawned by known installer services like msiexec.exe or setup.exe from trusted vendor directories.
• Scenario: Execution of specialized utility scripts or small administrative tools developed by internal IT teams that were packed with Crunch PE to optimize distribution size over slow networks.
  • Filter/Exclusion: Exclude processes located in internal IT tool directories (e.g., C:\ITTools\, C:\AdminUtils\) or exclude processes where the parent is a known administrative shell like powershell.exe or cmd.exe and the file hash matches a pre-approved list of internal tool hashes.
• Scenario: Scheduled maintenance jobs that run small, packed diagnostic scripts or health-check utilities (e.g., `diskcheck