PECompactv110b1

Hunt Hypothesis

This detection identifies potentially malicious or obfuscated executable files that utilize specific PE header compression signatures often employed by adversaries to evade static analysis and reduce file footprint. A proactive hunt for this indicator within Azure Sentinel is essential to uncover stealthy threats that may bypass traditional signature-based defenses, allowing the SOC team to investigate early-stage compromise indicators before they escalate into active incidents.

YARA Rule

rule PECompactv110b1
{
      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 28 63 40 ?? 87 DD 8B 85 AD 63 }

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 PECompactv110b1 detection rule, along with recommended filters or exclusions:

• Scenario: Microsoft Office Click-to-Run Updates

  • Context: The OfficeClickToRun.exe process frequently executes compressed PE binaries during background updates to patch Word, Excel, and PowerPoint. These executables often utilize the specific compression algorithm flagged by this rule as part of their standard delivery mechanism.
  • Filter/Exclusion: Exclude processes where the command line contains Click2Run.exe or the file path matches C:\Program Files\Microsoft Office*\root\Office16\*.

• Scenario: Scheduled Antivirus Definition Updates

  • Context: Enterprise endpoints running Symantec Endpoint Protection or McAfee Total Protection trigger compressed PE modules during their daily scheduled definition sync jobs. The update engine unpacks and executes these compacted signatures, mimicking the behavior of a malicious packed executable.
  • Filter/Exclusion: Create an exclusion for process names Symantec Antivirus.exe or mfevcservice.exe when the parent process is identified as the scheduled task runner (svchost.exe with specific service name).

• Scenario: Deployment of Enterprise Applications via SCCM

  • Context: During mass software deployments, Microsoft Endpoint Configuration Manager (SCCM) or Intune pushes application installers that are heavily compressed to reduce bandwidth usage. The installation agent (ccmsetup.exe) extracts and runs these compacted PE files, triggering the rule on every workstation receiving the update.
  • Filter/Exclusion: Filter out alerts where the parent process is ccmsetup.exe or wsman.exe (WinRM) and the file extension is .msi or .exe located within