RCryptorv16Vaska

Hunt Hypothesis

This detection targets the execution of the RCryptorv16Vaska ransomware variant, which adversaries often deploy to encrypt critical data and disrupt operations within Azure environments. Proactively hunting for this signature allows the SOC team to identify early-stage infection indicators before full encryption occurs, enabling rapid containment and minimizing potential business impact.

YARA Rule

rule RCryptorv16Vaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 33 D0 68 [4] FF D2 }
	$a1 = { 33 D0 68 [4] FF D2 B8 [4] 3D [4] 74 06 80 30 ?? 40 EB F3 }

condition:
		$a0 at pe.entry_point or $a1 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the RCryptorv16Vaska detection rule, including suggested filters and exclusions:

• Scenario: Enterprise Antivirus Real-Time Scanning

  • Context: The YARA rule may flag the real-time scanning engine of major security suites (e.g., Microsoft Defender, CrowdStrike Falcon) as a potential cryptor when they actively scan or decrypt files in user directories. These engines often utilize similar heuristic behaviors to RCryptorv16Vaska during file access.
  • Filter/Exclusion: Exclude the specific process names of known security agents from the detection scope.
    • Action: Add process_name exclusions for MsMpEng.exe, Cfssvc.exe, and CrowdStrikeService.exe.

• Scenario: Scheduled Backup Encryption Jobs

  • Context: Automated backup solutions like Veeam Backup & Replication or Rubrik often encrypt data blocks on disk before archiving. The RCryptorv16Vaska rule might misinterpret the encryption of large datasets by these scheduled jobs as a ransomware-like crypting event, especially when running during peak business hours.
  • Filter/Exclusion: Create an exclusion based on the service account and execution time window.
    • Action: Exclude processes running under the VeeamBRService or RubrikAgent service accounts between 02:00 and 06:00 local time, or filter by process path containing \Program Files\Veeam\.

• Scenario: Office Document Macro Execution

  • Context: Users frequently open complex Excel workbooks with embedded macros that perform data transformation and temporary file encryption (e.g., for secure reporting). The YARA signature might match the macro engine’s behavior when it modifies .xlsx files, triggering a