EXECryptor239DLLminimumprotection

Hunt Hypothesis

This detection identifies potential fileless or DLL-based execution of the EXECryptor239 malware family, which often leverages minimum protection settings to evade standard antivirus signatures and execute malicious payloads in memory. Proactively hunting for this behavior in Azure Sentinel is critical because low-severity alerts may be overlooked by automated triage, allowing adversaries to establish a foothold before escalating their attack through lateral movement or data exfiltration.

YARA Rule

rule EXECryptor239DLLminimumprotection
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 51 68 [4] 87 2C 24 8B CD 5D 81 E1 [4] E9 [3] 00 89 45 F8 51 68 [4] 59 81 F1 [4] 0B 0D [4] 81 E9 [4] E9 [3] 00 81 C2 [4] E8 [3] 00 87 0C 24 59 51 64 8B 05 30 00 00 00 8B 40 0C 8B 40 0C E9 [3] 00 F7 D6 2B D5 E9 [3] 00 87 3C 24 8B CF 5F 87 14 24 1B CA E9 [3] 00 83 C4 08 68 [4] E9 [3] 00 C3 E9 [3] 00 E9 [3] 00 50 8B C5 87 04 24 8B EC 51 0F 88 [3] 00 FF 05 [4] E9 [3] 00 87 0C 24 59 99 03 04 24 E9 [3] 00 C3 81 D5 [4] 9C E9 [3] 00 81 FA [4] E9 [3] 00 C1 C3 15 81 CB [4] 81 F3 [4] 81 C3 [4] 87 }

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 EXECryptor239DLLminimumprotection detection rule, including recommended filters and exclusions:

• Antivirus Engine Scanning of Compressed Archives

  • Scenario: The enterprise endpoint protection agent (e.g., Microsoft Defender Antivirus or CrowdStrike Falcon) performs routine on-access scanning of large .zip or .7z files containing executable payloads. The YARA rule triggers because the scanner’s internal process (MsMpEng.exe or Csfalcon.exe) loads a DLL to decrypt and analyze the archive contents, mimicking the behavior of an encryption tool.
  • Filter/Exclusion: Add a process exclusion for the specific AV engine executable (e.g., Path: C:\Program Files\Microsoft Defender\MsMpEng.exe or Process Name: MsMpEng.exe) to ignore detections originating from this parent process when accessing archive files.

• Scheduled Backup Job Executing Encryption Modules

  • Scenario: A nightly scheduled task (e.g., via Windows Task Scheduler) runs a backup solution like Veeam Agent for Microsoft Windows or Acronis Cyber Protect. During the “Encrypt” phase of the job, the service loads specific DLLs to apply AES-256 encryption to the backup stream, triggering the rule due to high I/O and memory mapping activities associated with the encryption module.
  • Filter/Exclusion: Create a time-based exclusion or a process-specific filter for the backup service executable (e.g., Process Name: VeeamAgent.exe or AcronisBackupService.exe) during the defined maintenance window (e.g., 02:00 – 04:00).

• Software Deployment via Microsoft Endpoint Configuration Manager (SCCM)

  • Scenario: The SCCM client (ccmexec.exe) dep