WinKriptv10MrCrimson

Hunt Hypothesis

This detection identifies the presence of the MrCrimson ransomware variant by matching file artifacts against its specific YARA signature within the Windows environment. Proactive hunting for this indicator is essential to confirm early-stage infection and validate endpoint coverage before the adversary initiates encryption or lateral movement activities in Azure Sentinel.

YARA Rule

rule WinKriptv10MrCrimson
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 33 C0 8B B8 00 [3] 8B 90 04 [3] 85 FF 74 1B 33 C9 50 EB 0C 8A 04 39 C0 C8 04 34 1B 88 04 39 41 3B CA 72 F0 58 83 C0 08 EB D5 61 E9 [4] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 }

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 5 specific false positive scenarios for the WinKriptv10MrCrimson detection rule, along with recommended filters and exclusions:

• Scenario: Enterprise Backup Agent Execution

  • Context: The backup software Veeam Backup & Replication or Rubrik initiates a scheduled job during business hours to encrypt database snapshots. These agents often utilize similar cryptographic libraries that match the MrCrimson signature pattern when processing large file sets on Windows servers.
  • Filter/Exclusion: Exclude processes running under the service accounts VeeamBackupSvc or RubrikAgentService. Additionally, filter out events where the parent process is vbr.exe (Veeam) or rubrik-agent.exe and the file path resides within dedicated backup directories (e.g., C:\ProgramData\Veeam\).

• Scenario: Automated Patching and Deployment Tools

  • Context: Microsoft Endpoint Configuration Manager (SCCM) or Ansible Tower executes a deployment task that involves encrypting application manifests or configuration files before distribution. The rule may trigger when these tools invoke PowerShell scripts to compress and encrypt payloads using native Windows Crypto APIs.
  • Filter/Exclusion: Create an exclusion for the process ccmexec.exe (SCCM) and ansible-playbook. Filter detections where the command line contains keywords like /encrypt, -Compress, or specific deployment tags, provided the execution occurs within a defined maintenance window (e.g., 02:00 – 04:00).

• Scenario: Document Management System Indexing

  • Context: The document management platform SharePoint Server or Alfresco runs its nightly indexing service. During this process, the system encrypts metadata and re-indexes large document libraries, which mim