PECompactv090

Hunt Hypothesis

This detection identifies potentially obfuscated or packed executable files that may conceal malicious payloads within their binary structure to evade static analysis. A proactive hunt is essential in Azure Sentinel to uncover stealthy threats that bypass traditional signature-based defenses by examining these compacted PE binaries for anomalous behavior before they execute on endpoints.

YARA Rule

rule PECompactv090
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 06 68 [2] 40 00 C3 9C 60 BD [2] 00 00 B9 02 00 00 00 B0 90 8D BD 7A 42 40 00 F3 AA 01 AD D9 43 40 00 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 PECompactv090 detection rule, along with recommended filters and exclusions:

• Scenario: Legitimate deployment of Microsoft Office updates via SCCM or Intune. The update installer often utilizes PE compression techniques to reduce payload size during installation, triggering the PECompact signature on files like Setup.exe or OfficeC2RClient.exe.

  • Filter/Exclusion: Exclude processes running under the Microsoft Office Click-to-Run Service (service name: ClickToRunSvc) and whitelist file paths within C:\Program Files\Microsoft Office\root\Office16\ or C:\Windows\System32\msiexec.exe when launched by SCCM agents.

• Scenario: Execution of third-party antivirus scanning engines (e.g., CrowdStrike Falcon, Carbon Black) performing real-time file analysis. These tools frequently compress temporary cache files or quarantine archives using PE compression before archiving them to disk.

  • Filter/Exclusion: Create an exclusion for the specific process names FalconSensor.exe (CrowdStrike), cbengine.exe (Carbon Black), and wofq.exe (Windows Defender) when they interact with files in temporary directories such as %TEMP% or C:\ProgramData\Microsoft\Windows Defender.

• Scenario: Scheduled nightly backup jobs utilizing Veeam Backup & Replication or Commvault. These agents often compress backup metadata and configuration manifests into PE-compressed executables before writing them to the backup repository.

  • Filter/Exclusion: Whitelist the process Veeam.Backup.Service.exe and CommServe.exe, specifically restricting the rule trigger to files located in the backup agent installation directory (e.g., C:\Program Files\Veeam\Backup and Replication Enterprise Manager) rather than user document