StonesPEEncruptorv113

Hunt Hypothesis

This hypothesis targets the presence of the StonesPEEncryptor v1.1.3 tool, a lightweight PE file encryptor often used by adversaries to obfuscate malicious payloads or protect implant binaries from static analysis. Proactively hunting for this specific version in Azure Sentinel allows the SOC to identify early-stage intrusion attempts or staging activities where attackers are preparing to deploy encrypted executables, enabling faster containment before the payload is decrypted and executed.

YARA Rule

rule StonesPEEncruptorv113
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 57 56 52 51 53 E8 [4] 5D 8B D5 81 }

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

• Legitimate Custom Build Pipeline: A CI/CD system (e.g., Jenkins or GitLab CI) executes a custom C++ compiler wrapper that applies a proprietary PE encryption layer to release binaries before packaging.
  • Filter/Exclusion: Allow files located in specific build artifact directories (e.g., C:\Builds\Artifacts\) or processes spawned by the CI agent service (e.g., jenkins-agent.exe, gitlab-runner.exe) where the parent process is a known build tool.
• Internal Utility Deployment: IT Operations deploys a custom internal diagnostic tool written in Rust or Go that uses a linker script to encrypt the PE header to obfuscate strings, and the binary is copied to C:\Program Files\InternalTools\.
  • Filter/Exclusion: Exclude files with a valid Authenticode signature from a trusted internal code-signing certificate (e.g., CN=Internal-Tools-CA) or files in the C:\Program Files\InternalTools\ directory path.
• Scheduled Maintenance Job: A scheduled task (Maintenance\PE-Optimizer) runs a legacy utility peopt.exe that re-encrypts the PE sections of application binaries to reduce size, triggered by the Task Scheduler service.
  • Filter/Exclusion: Allow execution if the parent process is svchost.exe (specifically the Task Scheduler service) and the target file resides in a designated maintenance folder (e.g., C:\Temp\PEOptimization\), or if the process command line contains --optimize and --encrypt.
• Development Sandbox Testing: A developer on a jump host or dev VM uses a tool like PEcrypt or a custom Python script to test encryption algorithms on sample binaries in a user profile directory.
  • Filter/Exclusion: Exclude files located under user profile