Upxv12MarcusLazlo

Hunt Hypothesis

This YARA rule targets the specific binary signature of the Upxv12MarcusLazlo malware, which is often associated with low-severity web server exploits or initial access payloads. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify dormant or low-and-slow infections that may have slipped past standard behavioral detections, ensuring early containment before lateral movement occurs.

YARA Rule

rule Upxv12MarcusLazlo
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 BE [4] 8D BE [4] 57 83 CD FF EB 05 A4 01 DB 75 07 8B 1E 83 EE FC 11 DB 72 F2 31 C0 40 01 DB 75 07 8B 1E 83 EE FC 11 DB 11 C0 01 DB 75 07 8B 1E 83 EE FC 11 DB 73 E6 31 C9 83 }

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

• Scenario: A developer or DevOps engineer uses the upx tool to compress a custom-built static binary (e.g., a Go or Rust application) to reduce its size for deployment to edge nodes or containers. The YARA rule Upxv12MarcusLazlo likely matches the specific version string or header structure of UPX 1.2, which is common in older CI/CD pipelines or legacy build scripts.

  • Filter/Exclusion: Exclude files located in specific build artifact directories (e.g., C:\Builds\artifacts\, /opt/builds/output/) or exclude processes where the parent process is a known build tool (e.g., msbuild.exe, gradle, make). Additionally, exclude if the file extension is .exe or .bin and the path contains vendor\ or third_party\.

• Scenario: An IT administrator runs a scheduled maintenance job that uses a custom script to compress and distribute a small utility tool (like a log collector or health check agent) to multiple servers. The tool is compressed with UPX 1.2 to fit within a specific size constraint for network transfer, triggering the rule on the target hosts during the copy/unpack phase.

  • Filter/Exclusion: Exclude if the file is being written by a service account associated with deployment (e.g., DOMAIN\svc-deploy) or if the parent process is a file transfer tool like robocopy.exe, scp, or rsync. Also, consider excluding if the file resides in a standard temporary extraction folder (e.g., C:\Temp\deploy\) and is accessed within 5 minutes of creation.

• Scenario: A legacy application or plugin (e.g., an old Java native library or a C++ DLL for a financial trading system) was compiled and packed with UPX