PECompactv110b6

Hunt Hypothesis

This detection identifies potentially malicious or obfuscated Portable Executable (PE) files that utilize specific compression signatures, which adversaries often employ to evade static analysis and hide embedded payloads within legitimate-looking binaries. Proactively hunting for these compressed artifacts in Azure Sentinel is essential to uncover stealthy command-and-control mechanisms or fileless attacks that may bypass traditional signature-based defenses due to their low severity classification and dynamic nature.

YARA Rule

rule PECompactv110b6
{
      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 ?? 00 87 DD 8B 85 9A 60 40 ?? 01 85 03 60 40 ?? 66 C7 85 ?? 60 40 ?? 90 90 01 85 92 60 40 ?? BB B7 }

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 PECompactv110b6 detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Office Click-to-Run Updater Execution

  • Context: The background service OfficeClickToRun.exe often spawns child processes that utilize PE compression techniques to minimize memory footprint during update checks. These legitimate binaries frequently match the structural signatures defined in the YARA rule.
  • Filter/Exclusion: Exclude processes where the parent image path contains \Microsoft Office ClickToRun\ and the process name is OfficeClickToRun.exe. Alternatively, add an exclusion for file paths matching C:\Program Files\Common Files\microsoft shared\ClickToRun\*.

• Scenario: Antivirus Real-Time Scanning Engine

  • Context: Endpoint protection agents like CrowdStrike Falcon or McAfee Enterprise often inject compressed PE modules into memory for real-time scanning. The rule may flag these internal engine components as suspicious due to their high compression ratio and specific header flags.
  • Filter/Exclusion: Create a whitelist exclusion based on the ImageName of known security vendors. For example, exclude any process where the image path starts with C:\Program Files\CrowdStrike\ or contains \McAfee\.

• Scenario: Scheduled System Backup Jobs (Veeam/Commvault)

  • Context: Enterprise backup solutions such as Veeam Agent or Commvault execute scheduled tasks that compress and package data using PE-based utility tools. During the nightly window, these tools generate temporary executables that trigger the PECompactv110b6 signature.
  • Filter/Exclusion: Implement a time-based filter to suppress alerts for this rule between 02:00 and 05