PECompactv092

Hunt Hypothesis

This detection identifies potentially obfuscated or packed executable files that utilize specific compression signatures to evade traditional signature-based scanning. A proactive hunt is essential in Azure Sentinel to uncover stealthy malware that may bypass initial ingestion filters by analyzing these compacted PE structures for anomalous behavior before they execute on endpoints.

YARA Rule

rule PECompactv092
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 06 68 [4] C3 9C 60 BD [4] B9 02 [3] B0 90 8D BD A5 4F 40 ?? F3 AA 01 AD 04 51 40 ?? FF B5 }

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 PECompactv092 detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Automated Deployment of Microsoft Office Updates via SCCM/Intune

  • Context: During scheduled maintenance windows, System Center Configuration Manager (SCCM) or Microsoft Intune deploys the latest Office 365 updates. The deployment agent often extracts and executes a compressed .msi or .exe payload that utilizes PE compression techniques to reduce download size, triggering this rule.
  • Suggested Filter/Exclusion: Create an exclusion based on the parent process name ccmsetup.exe (SCCM) or Microsoft.IntuneManagement.Extension.exe. Additionally, filter by file path containing \Program Files\Microsoft Office\Updates\ to ignore updates installed in known directories.

• Scenario: Execution of Compressed Installers by Chocolatey

  • Context: The IT team uses Chocolatey for software standardization. When installing applications like Adobe Acrobat or Zoom, the choco.exe service often downloads and executes compressed installer binaries (.nupkg extracted to .exe) that match the PECompact signature.
  • Suggested Filter/Exclusion: Exclude processes where the parent process is choco.exe running under the context of a specific service account (e.g., DOMAIN\svc-chocolatey). A file hash exclusion for known Chocolatey package binaries can also be applied to prevent recurring alerts on standard software.

• Scenario: Scheduled Antivirus Definition Updates

  • Context: Endpoint protection suites like CrowdStrike Falcon or Microsoft Defender perform daily definition updates. The update engine often downloads a compressed binary payload containing the new signatures, which is then executed as a PE-compact file to minimize network traffic and disk I/O.
  • **Suggested