PECompactv098

Hunt Hypothesis

This detection identifies potentially obfuscated or packed executable files that may conceal malicious payloads within their compressed structure. Proactively hunting for these artifacts in Azure Sentinel is essential to uncover stealthy threats that evade standard signature-based scanning and could indicate early-stage adversary activity.

YARA Rule

rule PECompactv098
{
      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 D7 84 40 ?? 87 DD 8B 85 5C 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 4 specific false positive scenarios for the PECompactv098 detection rule, along with targeted filters and exclusions:

• Scenario: Automated Deployment of Microsoft Office Updates via SCCM/Intune

  • Context: The System Center Configuration Manager (SCCM) or Microsoft Intune agent frequently downloads and executes compressed .msi or .exe payloads containing the PECompactv098 signature during scheduled maintenance windows. These legitimate installers often use this specific compression algorithm to reduce bandwidth usage.
  • Filter/Exclusion: Create a process exclusion for ccmsetup.exe (SCCM) and Microsoft.Workplace.JoinApp.exe (Intune) running under the SYSTEM or Network Service account, specifically when the file path contains \Program Files\Microsoft Intune Management Extension\.

• Scenario: Antivirus Real-Time Scanning of Compressed Archives

  • Context: Enterprise security suites like CrowdStrike Falcon or Symantec Endpoint Protection often extract and scan compressed archives (e.g., .zip, .7z) in real-time. The extraction engine generates temporary executable stubs that match the PECompactv098 signature before the actual payload is fully unpacked, triggering a detection on the temporary artifact rather than the final application.
  • Filter/Exclusion: Exclude files located within the C:\Windows\Temp and %TEMP% directories where the process name matches known AV service executables (e.g., FalconSensor.exe, Symantec Antivirus.exe). Additionally, add a rule to ignore alerts if the file age is less than 5 minutes.

• Scenario: Scheduled PowerShell Script Execution for Database Backups

  • Context: A nightly scheduled task runs a custom PowerShell script (BackupDB.ps1) that invokes a compressed utility