RCryptorv16xVaska

Hunt Hypothesis

This hunt detects the execution of the RCryptorv16xVaska malware variant to identify early-stage ransomware or file encryption activities that may evade standard signature-based detection. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to validate YARA rule efficacy and isolate potential threats before they escalate into full-scale data compromise events.

YARA Rule

rule RCryptorv16xVaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 90 61 61 80 7F F0 45 90 60 0F 85 1B 8B 1F FF 68 [4] C3 }

condition:
		$a0 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

Here are 4 specific false positive scenarios for the RCryptorv16xVaska detection rule in an enterprise environment, including suggested filters or exclusions:

• Scenario: Enterprise Antivirus Real-Time Scanning

  • Context: The endpoint protection agent (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint) performs real-time heuristic scanning of executable files. When the AV engine scans a large batch of newly downloaded installers or updates, its internal scanning process often mimics the memory footprint and behavioral patterns of the RCryptor malware family, triggering a false alert.
  • Suggested Filter/Exclusion: Create an exclusion rule for the specific process names of the security agent (e.g., C:\Program Files\CrowdStrike\fsqa.exe or C:\Windows\System32\msmpeng.exe) when they are executing scanning operations. Additionally, exclude the “Real-Time Protection” service PID from the YARA rule scope during high-volume update windows.

• Scenario: Scheduled Backup and Encryption Jobs

  • Context: Automated backup solutions (e.g., Veeam Agent, Acronis Cyber Protect) or document management tools (e.g., SharePoint Online Sync Client) frequently encrypt local cache files or compress large datasets during scheduled maintenance windows. These operations involve rapid file creation, encryption key generation, and memory allocation that closely resemble the RCryptor’s ransomware behavior of locking files.
  • Suggested Filter/Exclusion: Implement a time-based exclusion window (e.g., 02:00–04:00 local time) for known backup service accounts (e.g., DOMAIN\BackupSvc). Alternatively, add a YARA condition to exclude processes running under the specific Service Account names associated with these tools from triggering the alert.

• **Scenario: Software Deployment and Patching