AsCryptv01SToRM4

Hunt Hypothesis

This detection identifies potential fileless or memory-based malware activity by leveraging the AsCryptv01SToRM4 YARA signature to scan for specific cryptographic artifacts indicative of early-stage encryption ransomware or data exfiltration tools. Proactive hunting is essential in Azure Sentinel to uncover low-severity indicators that may represent initial reconnaissance phases, allowing the SOC team to investigate and contain threats before they escalate into full-scale incidents.

YARA Rule

rule AsCryptv01SToRM4
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 80 [3] 83 [4] 90 90 90 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 5 specific false positive scenarios for the AsCryptv01SToRM4 detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Microsoft Defender Antivirus Real-Time Scanning of Encrypted Archives

  • Context: The YARA rule flags the scanning process when MsMpEng.exe (Microsoft Defender) automatically scans large .7z or encrypted .zip files containing nested archives in the user’s “Downloads” folder. The heuristic signature matches the encryption header structure defined in AsCryptv01SToRM4, mistaking legitimate archive processing for a potential steganographic or encryption anomaly.
  • Suggested Filter/Exclusion: Add an exclusion to the detection logic that ignores alerts where the parent process is MsMpEng.exe and the file extension ends in .7z, .zip, or .rar. Alternatively, configure Microsoft Defender’s “Real-time protection” exclusions for the specific directory paths (e.g., C:\Users\*\Downloads) to bypass this specific signature check.

• Scenario: Scheduled Backup Job Executing via Veeam Agent

  • Context: A nightly scheduled task runs VeeamAgent.exe which compresses and encrypts local system state backups before uploading them to the cloud repository. The rule triggers because the backup software utilizes a custom encryption block structure that overlaps with the AsCryptv01SToRM4 signature, specifically during the “pre-encryption” phase of the job execution.
  • Suggested Filter/Exclusion: Create a time-based exclusion window (e.g., 01:00 to 05:00 UTC) for alerts generated by VeeamAgent.exe. Additionally, filter out events where the command line arguments contain specific backup flags like /backup or /encrypt, ensuring only non-backup encryption