PECompactv133

Hunt Hypothesis

This detection identifies potentially obfuscated or packed executable files that may conceal malicious payloads to evade standard signature-based scanning. Proactively hunting for these artifacts in Azure Sentinel is essential to uncover stealthy threats that leverage compression techniques to bypass initial security controls and reduce their visibility during early-stage analysis.

YARA Rule

rule PECompactv133
{
      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 80 40 ?? 87 DD 8B 85 A6 80 40 ?? 01 85 03 80 40 ?? 66 C7 85 00 80 40 ?? 90 90 01 85 9E 80 40 ?? BB E8 0E }

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 PECompactv133 detection rule, along with targeted filters and exclusions:

• Scenario: Deployment of Microsoft Office Click-to-Run updates.

  • Context: The OfficeClickToRun.exe process frequently spawns compacted PE binaries during background update cycles to save disk space and improve load times.
  • Filter/Exclusion: Exclude processes where the parent image path matches C:\Program Files\Microsoft Office\root\Office16\* or specifically filter for OfficeClickToRun.exe as the parent process.

• Scenario: Execution of Microsoft System Center Configuration Manager (SCCM) client tasks.

  • Context: The SCCM Client (ccmexec.exe) often launches compacted installers and scripts during scheduled maintenance windows to distribute software packages across the enterprise.
  • Filter/Exclusion: Create a whitelist for ccmexec.exe running under the SYSTEM account, specifically when the process command line contains keywords like “SoftwareDistribution” or “AppDeployment”.

• Scenario: Automated antivirus definition updates via CrowdStrike Falcon.

  • Context: The FalconSensorService injects compacted PE modules to update its local threat intelligence database without requiring a full service restart, triggering the detection logic.
  • Filter/Exclusion: Exclude files located in C:\Program Files\CrowdStrike\ where the process name is falcon.sysmon.exe or csfalcon.exe.

• Scenario: Scheduled PowerShell jobs running legacy administrative scripts.

  • Context: IT Operations teams utilize scheduled tasks to run compacted .ps1 scripts wrapped in a PE container (often using tools like Ngen) for performance optimization during off-hours maintenance.
  • Filter/Exclusion: Filter out events