PECompactv123b3v1241

Hunt Hypothesis

This detection identifies potentially obfuscated or packed Portable Executable (PE) binaries that may conceal malicious payloads within their compressed structure. A proactive hunt is essential in Azure Sentinel to uncover stealthy adversaries who utilize packing techniques to evade traditional signature-based scanning and delay the discovery of embedded threats.

YARA Rule

rule PECompactv123b3v1241
{
      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 ?? D2 08 }

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 PECompactv123b3v1241 detection rule, tailored for a legitimate enterprise environment:

• Scenario: Automated Endpoint Protection Scans triggering on compressed update payloads.

  • Context: During nightly maintenance windows, Microsoft Defender for Endpoint or CrowdStrike Falcon agents download and unpack large security definition updates. These updates often utilize PE compression algorithms (like UPX or custom packers) that match the signature of PECompactv123b3v1241.
  • Suggested Filter: Exclude processes where the parent process is the specific antivirus engine service (e.g., MsMpEng.exe, CnsService.exe) and the file path contains \ProgramData\Microsoft\Windows Defender\ or \CrowdStrike\.

• Scenario: Deployment of compressed software packages via Configuration Management tools.

  • Context: When Microsoft Endpoint Configuration Manager (ConfigMgr/SCCM) or Ansible pushes a new application package to workstations, the installer executable is often pre-compiled with compression headers to reduce network bandwidth usage during distribution. These installers frequently trigger this rule upon execution.
  • Suggested Filter: Create an exclusion for processes launched by deployment agents (e.g., ccmexec.exe, ansible-runner) where the file path resides within the standard Software Center or Ansible cache directories (e.g., \Program Files\Microsoft Configuration Manager\).

• Scenario: Execution of compressed backup and archiving utilities.

  • Context: Scheduled jobs using tools like Veeam Backup & Replication or Acronis Cyber Protect often generate temporary, compressed PE executables to handle data chunking before writing to the repository. These temporary binaries may match the PECompact signature during the active backup window.