PECompactv184

Hunt Hypothesis

This detection identifies potentially malicious or obfuscated Portable Executable (PE) files utilizing the PECompact v184 compression technique, which adversaries often employ to evade signature-based analysis by reducing file size and hiding embedded payloads. A proactive hunt is essential in Azure Sentinel to uncover these compressed artifacts that may bypass initial scanning thresholds, allowing analysts to validate their legitimacy before they execute within the environment.

YARA Rule

rule PECompactv184
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 33 C0 8B C4 83 C0 04 93 8B E3 8B 5B FC 81 }

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 PECompactv184 detection rule, along with recommended filters or exclusions:

• Scenario: Automated deployment of the Microsoft Office Click-to-Run (C2R) update service.

  • Context: The OfficeClickToRun.exe process frequently utilizes compression algorithms that match the PECompact signature when downloading and applying cumulative updates in the background.
  • Filter/Exclusion: Exclude processes where the executable path contains \Microsoft Office ClickToRun\ and the parent process is Ctfmon.exe or OfficeClickToRun.exe.

• Scenario: Scheduled antivirus definition update by Symantec Endpoint Protection (SEP).

  • Context: The SEP LiveUpdate service (Rtvscan64.exe) often unpacks compressed definition packages that trigger the PECompact signature during its nightly maintenance window.
  • Filter/Exclusion: Create a whitelist rule for Rtvscan64.exe when running under the user context NT SERVICE\Symantec LiveUpdate.

• Scenario: Execution of the Windows System Center Configuration Manager (SCCM) client update task.

  • Context: The ccmexec.exe process compresses and stages application payloads before installation, which often mimics the PECompact structure detected by this rule.
  • Filter/Exclusion: Exclude events where the command line arguments contain /install or /update and the parent process is WUAHandler.exe.

• Scenario: Background execution of the Adobe Acrobat Reader “Auto-Update” service.

  • Context: The AdobeARM.exe (Acrobat Update Manager) frequently compresses temporary installation files to optimize disk I/O, triggering a match on the PECompact signature during off-hours.
  • Filter/Exclusion: Filter out alerts