PECompactv094

Hunt Hypothesis

This detection identifies potentially malicious or obfuscated executable files that match the specific structural patterns defined by the PECompactv094 YARA signature within the Azure Sentinel environment. A proactive hunt is essential to uncover hidden threats that may evade standard signature-based defenses, allowing analysts to investigate and isolate these compacted binaries before they can execute advanced attack chains.

YARA Rule

rule PECompactv094
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 06 68 [4] C3 9C 60 E8 [4] 5D 55 58 81 ED [4] 2B 85 [4] 01 85 [4] 50 B9 02 }

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

Here are 5 specific false positive scenarios for the PECompactv094 detection rule, along with suggested filters and exclusions:

• Scenario: Microsoft Visual Studio Installer Execution

  • Context: The vcredist.exe or msiexec.exe processes frequently load compacted PE binaries during routine .NET Framework or Visual C++ Redistributable updates. These installers often utilize compression techniques that match the rule’s signature for packed executables, triggering alerts when developers or automated patching systems run updates on workstations.
  • Suggested Filter/Exclusion: Exclude processes where ImageName is vcredist.exe, msiexec.exe, or Setup.exe AND ParentImageName contains Microsoft Visual Studio Installer. Additionally, filter by file path containing \Program Files\Microsoft Visual Studio\ or \Windows\System32\.

• Scenario: Antivirus Real-Time Scanning of Compressed Archives

  • Context: Enterprise Endpoint Detection and Response (EDR) agents like CrowdStrike Falcon, SentinelOne, or Carbon Black often spawn temporary helper processes to scan compressed archives (.zip, .7z) in real-time. These helpers frequently load compacted PE modules that mimic the behavior of packed malware, causing high-volume false positives during large file transfers or backup operations.
  • Suggested Filter/Exclusion: Create a whitelist exclusion for known EDR service processes (e.g., C:\Program Files\CrowdStrike\fsqa.exe, sentinelone-agent.exe) and their child processes when the CommandLine contains keywords like “scan”, “archive”, or “backup”.

• Scenario: Scheduled System Maintenance Jobs

  • Context: Automated maintenance tasks, such as Windows Update scheduled jobs or Sysmon log rotation scripts, often invoke