PECompactv200alpha38

Hunt Hypothesis

This detection identifies potentially malicious or obfuscated Portable Executable (PE) files that match the specific structural patterns defined by the PECompactv200alpha38 YARA signature, which often indicates the use of packing techniques to conceal code analysis. Proactively hunting for these artifacts in Azure Sentinel is essential because packed executables are frequently employed by adversaries to evade static analysis and hide malicious payloads within legitimate-looking binaries before they execute on endpoints.

YARA Rule

rule PECompactv200alpha38
{
      meta:
		author="malware-lu"
strings:
		$a0 = { B8 [4] 80 B8 BF 10 00 10 01 74 7A C6 80 BF 10 00 10 01 9C 55 53 51 57 52 56 8D 98 0F 10 00 10 8B 53 14 8B E8 6A 40 68 00 10 00 00 FF 73 04 6A 00 8B 4B 10 03 CA 8B 01 FF D0 8B F8 50 8B 33 8B 53 14 03 F2 8B 4B 0C 03 CA 8D 85 B7 10 00 10 FF 73 04 8F }

condition:
		$a0
}

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 PECompactv200alpha38 detection rule, along with recommended filters and exclusions:

• Scenario: Automated Office Document Macro Compilation

  • Context: When Microsoft Office (Excel/Word) processes documents containing complex macros or embedded objects, it often utilizes a temporary PE-compiled helper process to render content. This process frequently exhibits the specific header characteristics targeted by PECompactv200alpha38.
  • Filter/Exclusion: Exclude processes where the parent executable is EXCEL.EXE or WINWORD.EXE and the file path contains \AppData\Local\Temp\ or \Documents\. Additionally, filter out events where the process name matches mscorwex.exe running under the context of an Office user.

• Scenario: Endpoint Protection Heuristic Scanning

  • Context: Enterprise antivirus solutions like CrowdStrike Falcon or Microsoft Defender for Endpoint often spawn lightweight, compacted scanning agents to perform real-time heuristic analysis on new files. These agents are frequently built using the PECompact architecture to minimize memory footprint during high-volume scans.
  • Filter/Exclusion: Create an exclusion rule for processes with known benign hashes belonging to C:\Program Files\CrowdStrike\ or C:\ProgramData\Microsoft\Windows Defender\. Specifically, exclude any process named csagent.exe, MsMpEng.exe, or SenseService.exe that matches the YARA signature.

• Scenario: Scheduled System Patching and Deployment

  • Context: During nightly maintenance windows, tools like Ansible, SCCM (System Center Configuration Manager), or Chocolatey deploy updates using compacted installer packages. These deployment agents often extract temporary PE-compiled executables to apply patches without triggering full installation overhead