EXECryptorv1401

Hunt Hypothesis

This detection identifies potential file encryption activities characteristic of ransomware or data exfiltration attempts by monitoring for specific executable behaviors defined in the EXECryptorv1401 YARA rule. Proactive hunting is essential because early identification of these low-severity encryption events allows the SOC team to investigate and contain threats before they escalate into full-scale ransomware incidents within the Azure Sentinel environment.

YARA Rule

rule EXECryptorv1401
{
      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 }

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 EXECryptorv1401 detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Office Background Auto-Save & Encryption

  • Context: When users open large documents (e.g., .docx, .xlsx) in Microsoft 365, the background process OfficeClickToRun.exe or OneDrive.exe often encrypts temporary cache files and auto-save versions. This behavior matches the signature of an executable performing bulk file encryption operations typical of ransomware.
  • Filter/Exclusion: Exclude processes with ImageFileName containing *Microsoft Office*, *OneDrive.exe*, or *OfficeClickToRun.exe* where the parent process is a known Office application, specifically when the target files are located in %LOCALAPPDATA%\Temp or %USERPROFILE%\AppData\Local\Microsoft\Office.

• Scenario: Enterprise Backup Agent Scheduled Job

  • Context: During nightly maintenance windows (e.g., 02:00 AM), enterprise backup solutions like Veeam Agent, Rubrik, or Acronis Cyber Protect execute scheduled jobs that compress and encrypt local data before transmission. The rule may misinterpret the high-volume file modification and encryption by these agents as a malicious execution event.
  • Filter/Exclusion: Create an exclusion for processes running under specific service accounts (e.g., DOMAIN\svc_backup) or image paths containing \Program Files\Veeam, \Rubrik, or \Acronis. Additionally, filter out events occurring during the defined maintenance window (01:00–05:00) where the process command line contains keywords like “backup,” “snapshot,” or “sync.”

• Scenario: Antivirus Real-Time Scanning and Quarantine