RCryptorv16dVaska

Hunt Hypothesis

This YARA rule targets the RCryptor ransomware variant, specifically identifying the Vaska strain, which indicates an adversary is actively deploying or staging ransomware payloads to encrypt critical data. Proactively hunting for this signature in Azure Sentinel allows the SOC team to detect early-stage ransomware activity on endpoints, enabling rapid isolation and response before the encryption process completes and impacts business operations.

YARA Rule

rule RCryptorv16dVaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 90 61 61 80 7F F0 45 90 60 0F 85 1B 8B 1F FF 68 }
	$a1 = { 60 90 61 61 80 7F F0 45 90 60 0F 85 1B 8B 1F FF 68 [4] B8 [4] 90 3D [4] 74 06 80 30 ?? 40 EB F3 }

condition:
		$a0 at pe.entry_point or $a1
}

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: When deploying or updating internal line-of-business applications built with older versions of the .NET Framework (specifically 2.0 or 3.5) that utilize the System.Security.Cryptography namespace heavily, the resulting executable or DLL may contain byte patterns matching the YARA signature.

  • Filter/Exclusion: Exclude files located in specific application directories (e.g., C:\Program Files\InternalApp\) or filter by file extension (.exe, .dll) combined with a known publisher certificate (e.g., “Internal Corp”) to avoid flagging signed, trusted binaries.

• Scheduled Backup Encryption Jobs: Enterprise backup solutions (such as Veeam, Commvault, or Windows Server Backup) that use AES-256 encryption for backup chains may generate temporary encrypted blobs or helper executables that exhibit cryptographic header patterns similar to the RCryptorv16dVaska signature.

  • Filter/Exclusion: Exclude processes or files spawned by known backup service accounts (e.g., svc-backup, vsa-backup) or filter by parent process name (e.g., vscan.exe, commvault.exe) to distinguish legitimate backup encryption from ransomware activity.

• Third-Party Code Signing Tools: During the release cycle, developers or release engineers use code signing tools (such as SignTool, OSS CodeSign, or JFrog Xray) to sign artifacts. These tools may create temporary signed containers or intermediate files that contain cryptographic structures triggering the YARA rule.

  • Filter/Exclusion: Exclude files in temporary signing directories (e.g., C:\Temp\signing\, %TEMP%\codesign\) or filter by the presence of a valid .pfx or .cer certificate file in the same directory context, indicating a legitimate signing operation.

• **Java Crypt