PECompactv099

Hunt Hypothesis

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

YARA Rule

rule PECompactv099
{
      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 2F 85 40 ?? 87 DD 8B 85 B4 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 5 specific false positive scenarios for the PECompactv099 detection rule, along with suggested filters and exclusions:

• Scenario: Microsoft Office Click-to-Run Updates

  • Context: The OfficeClickToRun.exe process frequently generates compacted PE binaries during background update cycles to optimize disk space for new feature packs.
  • Filter/Exclusion: Exclude the file path C:\Program Files\Microsoft Office\root\Office16\*.exe and filter out processes with the command line containing --update.

• Scenario: Windows Defender Antivirus Engine Scans

  • Context: The Windows Defender engine (MsMpEng.exe) often creates temporary compacted PE files in its quarantine or cache directories while analyzing large archives, triggering the rule due to specific header compression signatures.
  • Filter/Exclusion: Create an exclusion for the directory C:\ProgramData\Microsoft\Windows Defender\Scans\History and whitelist the process image path C:\Program Files\Windows Defender\MsMpEng.exe.

• Scenario: System Center Configuration Manager (SCCM) Application Deployment

  • Context: During scheduled application deployments, SCCM (ccmsetup.exe) extracts compressed payloads which often utilize the PE Compactv099 signature structure before installation begins.
  • Filter/Exclusion: Exclude files located in C:\Windows\CCMCache and filter events where the parent process is ccmexec.exe.

• Scenario: Antivirus Endpoint Protection Scans (e.g., CrowdStrike or SentinelOne)

  • Context: Third-party EDR agents like CrowdStrike (csfalcon.exe) or SentinelOne (S1Service.exe) generate compacted PE logs and telemetry files during real-time scanning bursts, mimicking the detection logic.