PECompactv126b1v126b2

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 embedded payloads within compressed binaries. A SOC team should proactively hunt for these artifacts in Azure Sentinel because low-severity alerts on such compacted executables can serve as early indicators of fileless or packed malware campaigns that might otherwise bypass standard signature-based defenses.

YARA Rule

rule PECompactv126b1v126b2
{
      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 70 40 ?? 87 DD 8B 85 A6 70 40 ?? 01 85 03 70 40 ?? 66 C7 85 70 40 90 ?? 90 01 85 9E 70 40 BB ?? 05 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 PECompactv126b1v126b2 detection rule, along with targeted filters and exclusions:

• Scenario: Legitimate automated software updates via Microsoft Update or third-party patch management tools (e.g., SCCM/Intune) downloading and executing compressed installer packages.

  • Filter/Exclusion: Exclude processes where the parent process is wuauclt.exe (Windows Update), ccmexec.exe (SCCM), or msiexec.exe, specifically when the file path contains \AppData\Local\Microsoft\Windows\Temporary Internet Files or \ProgramData\Microsoft\Windows\DeliveryOptimization.

• Scenario: Execution of compressed deployment scripts by DevOps automation tools such as Ansible, Puppet, or Jenkins agents running on build servers.

  • Filter/Exclusion: Create a whitelist for the specific user accounts used by these agents (e.g., svc-ansible, jenkins-agent) and exclude any PE file execution originating from the directory C:\Program Files\Ansible or D:\Jenkins\workspace.

• Scenario: Scheduled backup jobs utilizing compression utilities like Veeam, Acronis, or native Windows Volume Shadow Copy Service (VSS) that generate temporary compressed archives.

  • Filter/Exclusion: Filter out events where the process name matches vss.exe, veeamqoreagent.exe, or acronis.agent.service and the file extension is .zip, .7z, or .cab.

• Scenario: System administrators manually running compressed diagnostic tools (e.g., Process Explorer, Sysinternals Suite) or extracting configuration archives during maintenance windows.

  • Filter/Exclusion: Exclude executions where the parent process is explorer.exe initiated by a user in