EXECryptorv153

Hunt Hypothesis

This detection rule identifies potential encryption-based ransomware activity by monitoring for specific executable behaviors associated with the EXECryptorv153 signature. Proactive hunting in Azure Sentinel is essential to uncover early-stage encryption events that may indicate an adversary attempting to secure data before full-scale deployment, allowing the SOC team to isolate affected assets and mitigate impact before widespread disruption occurs.

YARA Rule

rule EXECryptorv153
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 24 00 00 00 8B 4C 24 0C C7 01 17 00 01 00 C7 81 B8 00 00 00 00 [2] 00 31 C0 89 41 14 89 41 18 80 A1 C1 00 00 00 FE C3 31 C0 64 FF 30 64 89 20 CC C3 }

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 EXECryptorv153 detection rule, including targeted filters and exclusions based on common enterprise environments:

• Scenario: Antivirus Real-Time Scanning of Large Archives

  • Context: Enterprise-grade Endpoint Detection and Response (EDR) or Antivirus solutions frequently scan large compressed files (.zip, .7z) in real-time. When a user downloads a massive dataset archive, the AV engine may spawn multiple child processes to decompress and encrypt/scan the contents, mimicking the behavior of an encryption ransomware attack.
  • Tools: Microsoft Defender for Endpoint, Symantec Endpoint Protection, Carbon Black.
  • Suggested Filter: Exclude process paths containing C:\Program Files\Microsoft Defender\ or C:\ProgramData\McAfee\. Additionally, filter out events where the parent process is a known AV service (e.g., MsMpEng.exe, rtvscan64.exe) and the file extension being modified is .zip or .7z.

• Scenario: Scheduled Database Backup Jobs

  • Context: Automated backup agents often perform bulk encryption of database files during off-hours. These jobs create temporary encrypted snapshots before moving them to storage, triggering high-volume file modification events that resemble ransomware activity.
  • Tools: Veeam Backup & Replication, Commvault, SQL Server Agent Jobs.
  • Suggested Filter: Create an exclusion for processes running under the SYSTEM or specific backup service accounts (e.g., VeeamBackupSvc, CommServe) during defined maintenance windows (e.g., 02:00–04:00 UTC). Specifically, exclude file paths located within known backup directories like D:\Backups\ or E:\SQL_Data\.