EXECryptorv13045

Hunt Hypothesis

This detection identifies potential file encryption activities indicative of ransomware or data exfiltration attempts by monitoring for specific executable behaviors defined in the EXECryptorv13045 YARA signature. Proactive hunting within Azure Sentinel is essential to uncover early-stage lateral movement and encryption anomalies that may precede a full-scale incident, allowing the SOC team to mitigate threats before they escalate despite the current low severity classification.

YARA Rule

rule EXECryptorv13045
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 24 00 00 00 8B 4C 24 0C C7 01 17 00 01 00 C7 81 [7] 31 C0 89 41 14 89 41 18 80 A1 }
	$a1 = { E8 24 [3] 8B 4C 24 0C C7 01 17 ?? 01 ?? C7 81 [7] 31 C0 89 41 14 89 41 18 80 A1 }

condition:
		$a0 at pe.entry_point or $a1 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the EXECryptorv13045 detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Office Background Encryption Services

  • Context: The excel.exe or winword.exe process invokes the msocsp.dll component to perform automatic document encryption during background saves or OneDrive sync operations. This behavior often mimics the memory scanning and file modification patterns of a cryptor malware.
  • Filter/Exclusion: Add an exclusion for the specific parent-child relationship: ParentImage = "C:\Program Files\Microsoft Office*\root\Office16\excel.exe" where ChildImage contains msocsp.dll. Alternatively, exclude processes running under the user context of SYSTEM or NETWORK SERVICE when executing these specific DLLs.

• Scenario: Scheduled Antivirus Real-Time Scanning Jobs

  • Context: Enterprise-grade EDR solutions like CrowdStrike Falcon or Microsoft Defender for Endpoint run scheduled deep-scan jobs that actively encrypt and decrypt temporary cache files in the %TEMP% directory. The rule may flag the rapid file I/O and cryptographic API calls as suspicious execution behavior.
  • Filter/Exclusion: Exclude processes where ImageName matches known EDR agents (e.g., CcsSvc.exe, MsMpEng.exe) performing tasks within the path C:\ProgramData\Microsoft\Windows Defender\Scans. Implement a time-based filter to ignore alerts during defined maintenance windows (e.g., 02:00–04:00 UTC).

• Scenario: Automated Backup and Archive Tools

  • Context: Enterprise backup solutions such as Veeam Agent or Acronis Cyber Protect execute scheduled tasks that compress and encrypt large datasets on disk. The rule detects