PseudoSigner01CrunchPEHeuristicAnorganix

Hunt Hypothesis

This rule identifies potentially malicious or obfuscated executable files that exhibit heuristic characteristics associated with the PseudoSigner tool, such as specific PE structure anomalies or code patterns used to disguise malware. Proactively hunting for these artifacts in Azure Sentinel allows the SOC team to detect stealthy payloads that may evade traditional signature-based detections by leveraging known structural quirks of this specific obfuscation technique.

YARA Rule

rule PseudoSigner01CrunchPEHeuristicAnorganix
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 E8 0E 00 00 00 5D 83 ED 06 8B C5 55 60 89 AD [4] 2B 85 00 00 00 00 E9 }

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 build engineer uses 7-Zip or WinRAR to compress a large directory of source code or build artifacts into a single .zip or .rar archive. The heuristic may flag the high entropy and specific structural patterns of the compressed PE files within the archive as “CrunchPE” behavior, especially if the archive contains many small, similar executables or libraries.

  • Filter/Exclusion: Exclude files with extensions .zip, .rar, .7z, or .tar.gz from the YARA scan, or add a condition to ignore files where the MIME type or extension matches known archive formats.

• Scenario: An IT administrator runs Sysinternals Suite tools (specifically strings.exe, depgraph.exe, or procexp.exe) to analyze a running application or a suspicious binary. These tools often load and parse PE headers extensively in memory, which can trigger heuristic-based YARA rules that look for PE structural anomalies or specific header manipulations associated with packers or protectors.

  • Filter/Exclusion: Add an exclusion for processes originating from the Sysinternals directory (e.g., C:\Sysinternals\*) or for known Sysinternals executable names (strings.exe, depgraph.exe, procexp.exe).

• Scenario: A software vendor or internal team uses UPX (Ultimate Packer for eXecutables) to compress release binaries to reduce size before deployment. While UPX is a legitimate packer, its specific compression algorithm and header structure can be misidentified by heuristic YARA rules designed to catch malicious packers or “CrunchPE” variants, leading to false positives on signed, production-ready binaries.

  • Filter/Exclusion: Exclude binaries that have a valid digital signature from a trusted internal or vendor certificate authority, or specifically exclude