StonesPEEncryptorv10

Hunt Hypothesis

This rule identifies the presence of the Stones PE Encryptor, a tool frequently used by adversaries to obfuscate malicious payloads and evade static analysis during the execution phase. Proactively hunting for this indicator in Azure Sentinel allows the SOC to detect low-severity, stealthy encryption activities that may precede more complex post-exploitation actions, ensuring early visibility into potential supply chain or fileless attack vectors.

YARA Rule

rule StonesPEEncryptorv10
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 57 56 52 51 53 E8 [4] 5D 8B D5 81 ED 63 3A 40 ?? 2B 95 C2 3A 40 ?? 83 EA 0B 89 95 CB 3A 40 ?? 8D B5 CA 3A 40 ?? 0F B6 36 }

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 runs a custom build script that uses UPX (Ultimate Packer for eXecutables) or a similar commercial packer (e.g., ASPack, Petite) to compress and protect the binary of a new microservice or internal CLI tool before deployment. The packing process alters the PE header and section entropy in a way that matches the StonesPEEncryptorv10 signature.

  • Filter/Exclusion: Exclude files located in specific build artifact directories (e.g., C:\Builds\, D:\Artifacts\) or exclude processes originating from known build agents (e.g., jenkins-agent.exe, azure-pipelines-agent.exe). Additionally, you can whitelist specific file hashes or paths where packed binaries are expected to reside temporarily.

• Scenario: An IT administrator installs a legacy line-of-business application or a specialized vendor tool (e.g., certain versions of Adobe Creative Cloud plugins, older Java-based enterprise apps, or specific hardware drivers) that uses PE encryption or packing as a standard distribution format. The installer drops the encrypted executable to disk, triggering the YARA rule during the file creation event.

  • Filter/Exclusion: Create a whitelist for known vendor installation directories (e.g., C:\Program Files\VendorApp\, C:\Program Files (x86)\LegacyTool\). You can also exclude files with specific extensions (like .dll or .exe) if they are located in trusted vendor folders and have a valid digital signature from the known vendor.

• Scenario: A security team or application development group uses a tool like PEB (PE Builder) or a custom obfuscation script to create a test binary for penetration testing or to verify the effectiveness of their own detection rules. This test file is written to a temporary directory or a dedicated test folder, triggering