RCryptor v20 Vaska additional

Hunt Hypothesis

This rule detects the presence of RCryptor v20 Vaska, a ransomware variant that encrypts files to extort victims, indicating a potential compromise where an adversary has deployed destructive malware. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify early-stage infections or staging activities before the encryption process begins, enabling rapid containment and reducing the overall impact of the incident.

YARA Rule

rule RCryptor_v20_Vaska_additional: PEiD
{
    strings:
        $a = { F7 D1 83 F1 FF 6A 00 F7 D1 83 F1 FF 81 04 24 ?? 02 00 00 F7 D1 83 F1 FF 59 BA 32 21 ?? 00 F7 D1 83 F1 FF F7 D1 83 F1 FF 80 02 E3 F7 D1 83 F1 FF C0 0A 05 F7 D1 83 F1 FF 80 02 6F F7 D1 83 F1 FF 80 32 A4 F7 D1 83 F1 FF 80 02 2D F7 D1 83 F1 FF 42 49 85 C9 75 }
    condition:
        $a 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

• Legacy .NET Application Deployment: The RCryptor v20 Vaska family is known for targeting .NET applications, and the “additional” YARA rule likely detects specific obfuscation patterns or string signatures common in older, custom-built internal tools. Legitimate, long-standing internal line-of-business (LOB) applications (e.g., a custom inventory management system written in C# .NET Framework 4.x) may retain these specific byte sequences if they have not been recompiled or updated in years.

  • Filter/Exclusion: Exclude files located in specific application directories (e.g., C:\Program Files\InternalTools\) that have a Last Modified Time older than 2 years and a valid digital signature from the internal corporate code-signing certificate.

• Third-Party Plugin or Add-on: Many enterprise software suites (such as older versions of Adobe Creative Cloud plugins, specific ERP modules, or legacy database connectors) bundle proprietary, obfuscated .NET assemblies. If a vendor ships a plugin that uses a similar packing or obfuscation technique to RCryptor, the YARA rule may match the plugin’s DLL or EXE file during installation or update cycles.

  • Filter/Exclusion: Exclude files within known third-party vendor directories (e.g., C:\Program Files\VendorName\Plugins\) that are part of a known software package (identified by a specific ProductCode in the registry or a matching FileVersion in the file metadata) and are not executing from unusual locations like C:\Users\Public\ or C:\Temp\.

• Scheduled Maintenance Job with Obfuscated Script: Some IT teams use PowerShell or C# scripts for automated maintenance tasks (e.g., log rotation, database backup verification) that are obfuscated to hide proprietary logic or credentials. If the obfuscation tool used by the developer produces a binary structure similar to the RC