StonesPEEncryptorv113

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 initial stages of an attack. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to detect early-stage compromise or staging activities before the encrypted payload is executed or further propagated within the environment.

YARA Rule

rule StonesPEEncryptorv113
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 57 56 52 51 53 E8 [4] 5D 8B D5 81 ED 97 3B 40 ?? 2B 95 2D 3C 40 ?? 83 EA 0B 89 95 36 3C 40 ?? 01 95 24 3C 40 ?? 01 95 28 }

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 sysadmin uses a custom internal build script that invokes the stonespe utility (or a similar PE encryption wrapper) to obfuscate small, non-critical helper binaries or test payloads before deploying them to staging environments.

  • Filter/Exclusion: Exclude files located in specific development directories (e.g., C:\Dev\Builds\, C:\Temp\Obfuscated\) or exclude processes where the parent process is a known build tool (e.g., msbuild.exe, dotnet.exe, python.exe running a specific script path).

• Scenario: An application installer (e.g., for a niche internal tool or a legacy Java-based app) uses a PE encryptor to compress or protect its native DLLs during the installation phase, triggering the YARA rule on the extracted temporary files.

  • Filter/Exclusion: Exclude files with extensions like .tmp or .part that are located in standard Windows temporary directories (%TEMP%, C:\Windows\Temp) and are created by known installer processes (e.g., msiexec.exe, setup.exe, install.exe).

• Scenario: A security team performs a controlled test or validation of their YARA rule set by intentionally dropping a known sample of the StonesPEEncryptor v1.1.3 into a sandboxed test folder to verify detection coverage.

  • Filter/Exclusion: Exclude files located in a designated “YARA Test” or “Detection Validation” directory (e.g., C:\SOC\YaraTests\) or exclude files owned by a specific service account used for testing (e.g., svc-yara-test).

• Scenario: A third-party backup or archiving agent uses a PE encryption/obfuscation step to protect metadata or small configuration blobs within its local cache before syncing to the