AsCryptv01SToRM3

Hunt Hypothesis

This hunt hypothesis targets adversaries utilizing the specific malware signature identified by the AsCryptv01SToRM3 YARA rule to detect early-stage encryption or file modification activities that may indicate a ransomware precursor. Proactively hunting for this behavior in Azure Sentinel is essential because, despite its low severity classification, it can reveal stealthy initial access attempts before they escalate into critical incidents requiring immediate response.

YARA Rule

rule AsCryptv01SToRM3
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 80 [3] 83 [4] 90 90 90 51 [3] 01 00 00 00 83 [2] E2 }

condition:
		$a0
}

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 AsCryptv01SToRM3 detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Enterprise Backup Agent Encryption Jobs

  • Context: Scheduled nightly backup jobs using tools like Veeam Backup & Replication or Commvault often trigger file encryption events when archiving data to the repository. The YARA rule may misinterpret the bulk encryption of large database files (e.g., .vmdk, .vhd) as a ransomware-like activity.
  • Filter/Exclusion: Exclude processes running under the Veeam.Backup.Service.exe or Commvault.Client.exe service accounts. Additionally, filter out events where the encrypted file extension is known backup formats (.vbk, .bck, .dbs) rather than user document types like .docx or .xlsx.

• Scenario: Admin-Driven Mass File Migration via PowerShell

  • Context: System administrators frequently execute custom PowerShell scripts to migrate data between on-premise servers and cloud storage (e.g., Azure Blob Storage). These scripts often utilize the .NET System.Security.Cryptography library to encrypt files in transit or at rest, mimicking the behavior of ransomware encryption.
  • Filter/Exclusion: Create an exclusion for process images named powershell.exe when the parent process is a scheduled task (e.g., “Admin_Migration_Job”) or when the command line arguments contain specific keywords like -Encrypt, -Compress, or references to internal migration scripts.

• Scenario: Endpoint DLP and Rights Management Scanning

  • Context: Data Loss Prevention (DLP) solutions such as Microsoft Purview Information Protection or Symantec DLP automatically scan and encrypt sensitive documents containing P