RCryptorv16bv16cVaska

Hunt Hypothesis

This YARA rule targets the RCryptor ransomware variant, specifically identifying its distinctive code patterns to detect the presence of this encryption tool within the environment. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify potential ransomware staging or early execution phases before the adversary initiates widespread data encryption.

YARA Rule

rule RCryptorv16bv16cVaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 8B C7 03 04 24 2B C7 80 38 50 0F 85 1B 8B 1F FF 68 }
	$a1 = { 8B C7 03 04 24 2B C7 80 38 50 0F 85 1B 8B 1F FF 68 [4] B8 [4] 3D [4] 74 06 80 30 ?? 40 EB F3 }

condition:
		$a0 at pe.entry_point or $a1 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Legacy .NET Application Deployment: A specific internal line-of-business (LOB) application built with an older version of the .NET Framework (e.g., 3.5 or 4.0) may contain obfuscated or packed assemblies that match the byte patterns of RCryptor v16. This often occurs when IT teams deploy updated versions of legacy desktop apps via Group Policy Objects (GPO) to user workstations.

  • Filter/Exclusion: Exclude files located in specific application directories (e.g., C:\Program Files\InternalLOBApp\) or exclude processes launched by the known deployment agent (e.g., ccmsetup.exe or msiexec.exe) if the detection is triggered on the file path rather than the running process.

• Third-Party Java/Scala Application Packaging: Enterprise applications developed in Java or Scala and packaged using tools like ProGuard or specific custom packers may inadvertently produce binary signatures that overlap with RCryptor v16, particularly if the application uses specific encryption libraries for local data storage. This is common in financial reporting tools or internal dashboards deployed via Docker or native installers.

  • Filter/Exclusion: Exclude detections for files with extensions .jar, .war, or .zip that are extracted to temporary directories (%TEMP%, %LOCALAPPDATA%) by known service accounts, or exclude specific known-good hashes of the application binaries.

• Scheduled Backup or Archiving Jobs: Enterprise backup solutions (e.g., Veeam, Commvault, or Acronis) or internal archiving scripts may compress and encrypt data chunks using algorithms that result in file headers matching the RCryptor v16 signature. This can happen if the backup agent creates temporary encrypted cache files on the agent host before transferring them to the repository.

  • Filter/Exclusion: Exclude detections for files created by known backup service