PECompactv100

Hunt Hypothesis

This detection identifies potentially obfuscated or packed executable files that may conceal malicious payloads within their structure to evade standard signature-based analysis. Proactively hunting for these compacted binaries in Azure Sentinel is essential because attackers frequently utilize packing techniques to bypass initial security controls and establish a foothold before executing their primary attack logic.

YARA Rule

rule PECompactv100
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 06 68 [4] C3 9C 60 E8 02 [3] 33 C0 8B C4 83 C0 04 93 8B E3 8B 5B FC 81 EB C4 84 40 ?? 87 DD 8B 85 49 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

Here are 4 specific false positive scenarios for the PECompactv100 detection rule, which identifies Portable Executable (PE) files utilizing the UPX or similar packing/compression techniques often associated with obfuscation:

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

  • Context: The System Center Configuration Manager (SCCM) or Microsoft Intune agent pushes cumulative updates for Office 365. These deployment packages are frequently compressed using UPX to reduce bandwidth usage during distribution, triggering the rule when the installer extracts and executes on endpoints.
  • Suggested Filter/Exclusion: Exclude processes running under the C:\Windows\CCM\ directory tree (specifically ccmsetup.exe, wuauserv.exe) or filter by the specific file hash of known Office update installers (*.msi or *.exe with a known SHA-256 signature).

• Scenario: Execution of Third-Party Antivirus Definition Updates

  • Context: Security vendors like CrowdStrike, SentinelOne, or Symantec deploy daily definition updates. The update executables are often packed to minimize storage footprint and speed up the installation process, causing the YARA rule to flag them as potentially suspicious packed binaries.
  • Suggested Filter/Exclusion: Create a whitelist for processes originating from specific vendor paths (e.g., C:\Program Files\CrowdStrike\ or C:\ProgramData\McAfee\) and exclude files with the publisher certificate “CrowdStrike, Inc.” or “Symantec Corporation”.

• Scenario: Scheduled Backup Jobs Using Compressed Archives

  • Context: Enterprise backup solutions such as Veeam Agent or Acronis Cyber Protect run scheduled jobs that extract compressed temporary executables to verify integrity before archiving. These temporary extraction tools are often