RCryptor v16 Vaska additional

Hunt Hypothesis

This hypothesis targets the presence of RCryptor v16 Vaska, a known ransomware variant, by identifying its specific code patterns to detect early-stage deployment or staging activities. Proactively hunting for this signature allows the SOC team to isolate affected assets before the encryption process begins, minimizing potential data loss and operational downtime in the Azure environment.

YARA Rule

rule RCryptor_v16_Vaska_additional: PEiD
{
    strings:
        $a = { 33 D0 68 ?? ?? ?? ?? FF D2 B8 ?? ?? ?? ?? 3D ?? ?? ?? ?? 74 06 80 30 ?? 40 EB F3 }
    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

• Scenario: A developer or DevOps engineer uses RCryptor to encrypt a proprietary configuration script or a small utility binary before packaging it for deployment in a CI/CD pipeline (e.g., via Jenkins or GitHub Actions). The encrypted artifact is temporarily stored in a build agent’s workspace or a shared network drive, triggering the YARA signature on the file system.

  • Filter/Exclusion: Exclude files located in standard CI/CD build directories (e.g., C:\jenkins\workspace\, /home/ci/builds/) or files with specific extensions like .enc or .bin that are explicitly marked as build artifacts. Additionally, exclude processes spawned by known CI agents (e.g., jenkins.exe, agent.exe) from generating new encrypted files.

• Scenario: An IT administrator uses RCryptor to protect a sensitive PowerShell script or a custom .NET assembly containing credentials or logic before distributing it to endpoints via Group Policy or SCCM (System Center Configuration Manager). The encrypted file is staged in a distribution point or a local admin temp folder during the deployment process.

  • Filter/Exclusion: Exclude files created by known deployment tools such as ccmsetup.exe, dcmsetup.exe, or gpupdate.exe. Also, consider excluding files in standard administrative temp directories (e.g., C:\Windows\Temp\, C:\Users\Public\Temp\) if the file size is below a certain threshold (e.g., < 5MB) and was created within the last 24 hours.

• Scenario: A security team or application developer uses RCryptor to encrypt a test binary or a sample payload for internal threat hunting or vulnerability testing. The encrypted file is placed in a dedicated test environment folder (e.g., C:\ThreatHunt\)