AsCryptv01SToRM2

Hunt Hypothesis

This detection identifies the presence of the AsCrypt ransomware variant (AsCryptv01SToRM2) by leveraging a specific YARA signature to flag potential initial infection or dormant malware artifacts within the environment. A proactive hunt is essential because early identification of this low-severity indicator allows the SOC team to isolate affected assets before the adversary escalates to active encryption and data exfiltration phases.

YARA Rule

rule AsCryptv01SToRM2
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 80 [3] 83 [4] 90 90 90 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 AsCryptv01SToRM2 detection rule, along with recommended filters and exclusions:

• Scenario: Microsoft Office Suite Auto-Save & Co-Authoring

  • Context: When multiple users collaborate on a shared document (e.g., Excel or PowerPoint) stored on a network share, the background process excel.exe or powerpnt.exe frequently creates temporary encrypted files in the %TEMP% directory to handle version locking and co-authoring states. The YARA rule may misinterpret these rapid, small file creations as suspicious encryption activity typical of ransomware.
  • Filter/Exclusion: Exclude processes running under the C:\Program Files\Microsoft Office\root\Office16\ path where the parent process is winword.exe, excel.exe, or powerpnt.exe. Specifically, filter out file creation events where the destination directory contains \AppData\Local\Temp\~ and the file extension is .tmp or .tmp~.

• Scenario: Windows Defender Scheduled Full Scan

  • Context: During a scheduled full system scan (triggered via Group Policy), MsMpEng.exe scans and temporarily locks/encrypts metadata for millions of files. The rule might flag the high volume of file access and temporary encryption of quarantine logs as an anomalous “mass encryption” event, triggering a false alert during maintenance windows.
  • Filter/Exclusion: Create an exclusion based on the process name MsMpEng.exe (Windows Defender Antivirus Service). Additionally, apply a time-based filter to suppress alerts generated between 02:00 and 06:00 UTC on weekdays, assuming this is the standard maintenance window.

• Scenario: Enterprise Backup Agent Operations (Veeam/Commvault) *