PECompactv110b2

Hunt Hypothesis

This detection identifies potentially malicious or obfuscated Portable Executable (PE) files that utilize specific compression signatures, which often indicate an adversary attempting to evade static analysis by hiding code within compressed binaries. A SOC team should proactively hunt for these artifacts in Azure Sentinel to uncover stealthy execution patterns that may bypass standard signature-based defenses and reveal early-stage fileless or packed malware campaigns.

YARA Rule

rule PECompactv110b2
{
      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 60 40 ?? 87 DD 8B 85 94 60 }

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 PECompactv110b2 detection rule, which targets potentially suspicious PE file packing or compression techniques:

• Scenario: Automated Antivirus Definition Updates

  • Context: Enterprise endpoint protection suites (e.g., Microsoft Defender, CrowdStrike Falcon) frequently download and install updated definition databases. These updates often arrive as compressed, packed executables to minimize bandwidth usage during the nightly update cycle. The YARA rule may flag these legitimate update binaries due to their high entropy and specific compression headers.
  • Filter/Exclusion: Create a path-based exclusion for the vendor’s installation directory (e.g., C:\Program Files\Microsoft Defender Antivirus\ or C:\ProgramData\CrowdStrike\) and exclude processes matching the executable name of the updater service (e.g., MsMpSvc.exe, csfalcon.exe).

• Scenario: Scheduled Software Deployment via SCCM/Intune

  • Context: During scheduled maintenance windows, System Center Configuration Manager (SCCM) or Microsoft Intune deploys new applications to endpoints. The deployment agents often extract and execute packed installer packages (.msi/.exe) that utilize the PECompact algorithm to reduce payload size before installation begins. This triggers the rule during the initial extraction phase of the deployment agent.
  • Filter/Exclusion: Exclude processes spawned by known deployment engines, such as ccmsetup.exe, intune-management-extension.exe, or wuauserv.exe. Additionally, apply a time-based filter to ignore alerts occurring only between 01:00 and 05:00 local time when bulk deployments are active.

• Scenario: Database Backup and Archiving Jobs

  • Context: Enterprise database administrators run nightly jobs using tools like Veeam, Commvault, or native SQL Server agents to