PECompactv140b5v140b6

Hunt Hypothesis

This detection identifies potentially obfuscated or packed executable files that may conceal malicious payloads through compression or encryption techniques to evade static analysis. Proactively hunting for these artifacts in Azure Sentinel is essential because attackers frequently employ packing mechanisms to bypass signature-based defenses and delay discovery during the initial execution phase.

YARA Rule

rule PECompactv140b5v140b6
{
      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 0F A0 40 ?? 87 DD 8B 85 A6 A0 40 ?? 01 85 03 A0 40 ?? 66 C7 85 ?? A0 40 ?? 90 90 01 85 9E A0 40 ?? BB 8A 11 }

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 3-5 specific false positive scenarios for the PECompactv140b5v140b6 detection rule, formatted with specific enterprise contexts and recommended filters:

• Scenario: Microsoft Office Click-to-Run Updates

  • Context: The OfficeClickToRun.exe process frequently executes compacted PE binaries during scheduled update cycles (e.g., via the “Microsoft Office 365” service) to patch Word, Excel, or Outlook. These updates often utilize compression techniques that match the signature of the rule.
  • Filter/Exclusion: Exclude processes where Process_Name equals OfficeClickToRun.exe and the file path contains \Program Files\Microsoft Office\root\Office16\. Additionally, filter out events occurring during standard maintenance windows (e.g., 02:00–04:00 local time).

• Scenario: Antivirus Engine Scanning of Compressed Archives

  • Context: Enterprise Endpoint Detection and Response (EDR) agents like CrowdStrike Falcon or SentinelOne often scan .zip or .7z archives containing compressed executables. When the agent extracts these files in memory for analysis, the resulting PE structure triggers the PECompactv140b5v140b6 signature as a potential anomaly.
  • Filter/Exclusion: Create an exclusion rule that ignores alerts where the parent process is the specific EDR service (e.g., FalconSensor.exe, SentinelOneAgent.exe) and the file extension of the scanned object is .zip, .7z, or .cab.

• Scenario: Deployment of Compressed Application Installers via SCCM/Intune

  • Context: System Center Configuration Manager (SCCM) or Microsoft Intune frequently deploys application