RCryptorv11Vaska

Hunt Hypothesis

This detection identifies the presence of the RCryptorv11Vaska ransomware variant on endpoints by matching file signatures against a specific YARA rule within Azure Sentinel. Proactive hunting for this threat is essential to confirm early-stage infection indicators and initiate rapid containment before the adversary progresses to data encryption or lateral movement phases.

YARA Rule

rule RCryptorv11Vaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 8B 04 24 83 E8 4F 68 [4] FF D0 }
	$a1 = { 8B 04 24 83 E8 4F 68 [4] FF D0 B8 [4] 3D [4] 74 06 80 30 ?? 40 EB F3 }

condition:
		$a0 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

Here are 4 specific false positive scenarios for the RCryptorv11Vaska detection rule, including suggested filters and exclusions tailored for a legitimate enterprise environment:

• Scenario: Enterprise Antivirus Engine Updates

  • Context: The RCryptorv11Vaska signature often detects file modification patterns similar to ransomware when the endpoint protection suite (e.g., CrowdStrike Falcon, SentinelOne, or Microsoft Defender) performs a background definition update. These engines frequently write new signature databases to disk using encryption routines that mimic the rule’s logic.
  • Suggested Filter/Exclusion: Create an exclusion for processes running under the vendor’s service account (e.g., SYSTEM or specific service accounts like CrowdStrikeService) and exclude file paths within the antivirus installation directory (e.g., C:\Program Files\CrowdStrike\). Additionally, filter out events where the process name matches known AV update executables such as csfalcon.exe, SentinelOneAgent.exe, or MsMpEng.exe.

• Scenario: Scheduled Backup and Archiving Jobs

  • Context: Enterprise backup solutions like Veeam, Rubrik, or Acronis Cyber Protect often compress and encrypt data during scheduled windows. The rule may trigger when these tools process large volumes of user documents, interpreting the bulk encryption operations as ransomware activity.
  • Suggested Filter/Exclusion: Implement a time-based exclusion for backup windows (e.g., 02:00–06:00 local time) or exclude specific service accounts used by backup agents (e.g., VeeamBackupService). Furthermore, add a path exclusion for known backup staging directories such as C:\ProgramData\Veeam\ or network shares designated for backups (\\BackupServer\Staging\).