WerusCrypter10Kas

Hunt Hypothesis

This hypothesis targets the presence of the WerusCrypter10Kas malware, a low-severity crypter often used to obfuscate payloads or establish initial footholds in compromised environments. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify stealthy, low-noise infections that may evade standard behavioral detections, ensuring early containment of crypters that could facilitate further lateral movement or data exfiltration.

YARA Rule

rule WerusCrypter10Kas
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 68 98 11 40 00 6A 00 E8 50 00 00 00 C9 C3 ED B3 FE FF FF 6A 00 E8 0C 00 00 00 FF 25 80 10 40 00 FF 25 84 10 40 00 FF 25 88 10 40 00 FF 25 8C 10 40 00 FF 25 90 10 40 00 FF 25 94 10 40 00 FF 25 98 10 40 00 FF 25 9C 10 40 00 FF 25 A0 10 40 00 FF 25 A4 10 40 00 FF 25 A8 10 40 00 FF 25 B0 10 40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 BB E8 12 40 00 80 33 05 E9 7D FF FF FF }

condition:
		$a0
}

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

• Legacy Application Maintenance: An IT administrator updates a legacy internal tool (e.g., a custom Java-based inventory system) that uses the Werus Crypter to obfuscate its native helper binaries to avoid antivirus signature detection.
  • Filter/Exclusion: Exclude files located in the specific application directory (e.g., C:\Apps\LegacyInventory\bin\) or exclude processes spawned by the application’s main executable (e.g., LegacyApp.exe) from triggering the YARA rule.
• Scheduled Backup Job: A nightly scheduled task runs a proprietary backup utility that uses the Werus Crypter to encrypt and compress backup archives before transferring them to the NAS. The YARA scan runs on the temporary staging folder where the encrypted files reside.
  • Filter/Exclusion: Exclude files matching the specific backup extension (e.g., .wcr or .enc) within the designated backup staging path (e.g., D:\BackupStaging\) or exclude the specific backup service account (e.g., svc-backup) from the detection scope.
• Development Environment Testing: A developer on the QA team is testing a new feature in a local development environment (e.g., Visual Studio Code or IntelliJ) that uses the Werus Crypter to generate test payloads for security validation. The YARA scanner picks up the generated test files in the user’s local workspace.
  • Filter/Exclusion: Exclude files within the developer’s local workspace directories (e.g., C:\Users\<devuser>\Projects\) or exclude processes associated with development IDEs (e.g., code.exe, idea64.exe) from the YARA rule evaluation.
• Third-Party Plugin Installation: An admin installs a third-party plugin for a CMS (e.g., WordPress or Drupal) that bundles a helper script encrypted with the Werus Crypter for performance optimization.