PECrypt15BitShapeSoftware

Hunt Hypothesis

This rule identifies potentially obfuscated or packed executable files using specific 15-bit shape characteristics, which adversaries may employ to hide malicious payloads from static analysis. Proactively hunting for these artifacts in Azure Sentinel helps detect early-stage fileless or memory-resident malware that traditional signature-based detections might miss.

YARA Rule

rule PECrypt15BitShapeSoftware
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 81 ED 55 20 40 00 B9 7B 09 00 00 8D BD 9D 20 40 00 8B F7 AC [48] AA E2 CC }

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 runs a local version of BitScribe (or similar 15-bit shape optimization tool) on a workstation to process CAD files or optimize 3D models before uploading them to the asset management system.
  • Filter/Exclusion: Exclude processes where the parent process is a known IDE (e.g., code.exe, idea64.exe) or a build tool (e.g., msbuild.exe, gradle) and the file path contains C:\Users\{User}\Projects\ or D:\Builds\.
• Scenario: An IT administrator deploys a custom PowerShell script using PECrypt to encrypt sensitive configuration files (e.g., appsettings.json or license keys) on a server prior to a maintenance window. The script invokes the PECrypt15BitShapeSoftware binary directly from a shared admin share.
  • Filter/Exclusion: Exclude executions where the command line arguments contain --encrypt or --sign and the image path is located in a known admin tool directory (e.g., C:\AdminTools\PECrypt\ or \\FileServer\Admin\Tools\).
• Scenario: A data science team uses BitShape software in a Jupyter Notebook environment to perform shape analysis on legacy 15-bit vector data. The kernel spawns the executable as a subprocess during a scheduled data pipeline job.
  • Filter/Exclusion: Exclude processes where the parent process is python.exe or jupyter-notebook.exe and the working directory matches the data science project path (e.g., C:\DataScience\Projects\LegacyVectors\).
• Scenario: A QA engineer runs a regression test suite that includes a unit test for the PECrypt library, which spawns the `PECrypt15Bit