WerusCrypter10byKas

Hunt Hypothesis

This hypothesis targets the presence of the WerusCrypter10byKas malware, a known cryptor often used in ransomware attacks or for data exfiltration, by leveraging YARA signatures to identify infected hosts. Proactively hunting for this specific variant in Azure Sentinel allows the SOC to detect early-stage infections or dormant payloads before they execute, thereby reducing the mean time to containment and preventing potential data encryption or loss.

YARA Rule

rule WerusCrypter10byKas
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BB E8 12 40 00 80 33 05 E9 7D FF FF FF }

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 security team or incident responder performs a manual memory dump of a suspected compromised host using a tool like Volatility or WinDbg, where the YARA rule scans the resulting memory image or the tool’s executable itself. The WerusCrypter10byKas signature may match specific byte patterns in the tool’s code or the captured memory segments, triggering a false positive.

  • Filter/Exclusion: Exclude processes where the parent process is volatility.exe, windbg.exe, or falconctl.exe (CrowdStrike Falcon) and the file path resides in the security tooling directory (e.g., C:\Tools\Security\ or C:\Program Files\CrowdStrike\).

• Scenario: An enterprise deployment of a legacy Java-based application or a custom internal tool uses a specific version of the Java Runtime Environment (JRE) or a bundled native library that contains a byte sequence matching the WerusCrypter10byKas signature. This often happens when the application is packaged with older, unpatched dependencies that have known cryptographic or compression routines resembling the crypter’s structure.

  • Filter/Exclusion: Exclude alerts where the file extension is .jar, .class, or .so and the file path contains jre, java, or lib directories, specifically for known internal applications (e.g., C:\Apps\InternalERP\lib\).

• Scenario: A scheduled maintenance job runs a custom Python script that uses the pycryptodome or cryptography library to perform data encryption/decryption for log rotation or backup preparation. The YARA rule may match the compiled Python bytecode (.pyc files) or the native C extensions (.pyd or .so files) of these libraries if they contain specific function signatures or data