EXECryptor239minimumprotection

Hunt Hypothesis

This hunt hypothesis targets the execution of potentially malicious binaries identified by the EXECryptor239 signature to detect early-stage encryption or file modification activities often associated with ransomware or fileless attacks. Proactively hunting for this behavior in Azure Sentinel is critical because its low severity classification may cause it to be overlooked during routine monitoring, necessitating manual investigation to confirm whether these executions represent legitimate operations or the initial phases of a stealthy adversary campaign.

YARA Rule

rule EXECryptor239minimumprotection
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 68 [4] E9 [3] FF 50 C1 C8 18 89 05 [4] C3 C1 C0 18 51 E9 [3] FF 84 C0 0F 84 6A F9 FF FF E9 [3] FF C3 E9 [3] FF E8 CF E9 FF FF B8 01 00 00 00 E9 [3] FF 2B D0 68 A0 36 80 D4 59 81 C9 64 98 FF 99 E9 [3] FF 84 C0 0F 84 8E EC FF FF E9 [3] FF C3 87 3C 24 5F 8B 00 03 45 FC 83 C0 18 E9 [3] FF 87 0C 24 59 B8 01 00 00 00 D3 E0 23 D0 E9 02 18 00 00 0F 8D DB 00 00 00 C1 E8 14 E9 CA 00 00 00 9D 87 0C 24 59 87 1C 24 68 AE 73 B9 96 E9 C5 10 00 00 0F 8A [4] E9 [3] FF 81 FD F5 FF 8F 07 E9 4F 10 00 00 C3 E9 5E 12 00 00 87 3C 24 E9 [3] FF E8 [3] FF 83 3D [4] 00 0F 85 [4] 8D 55 EC B8 [4] E9 [3] FF E8 A7 1A 00 00 E8 2A CB FF FF E9 [3] FF C3 E9 [3] FF 59 89 45 E0 }

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 4 specific false positive scenarios for the EXECryptor239minimumprotection detection rule, including suggested filters and exclusions:

• Scenario: Antivirus Real-Time Scanning of Large Archives

  • Context: Enterprise Endpoint Detection and Response (EDR) agents like CrowdStrike Falcon or Microsoft Defender for Endpoint often trigger this rule when scanning large .zip or .7z archives containing executable payloads. The YARA signature may misinterpret the internal encryption of the archive structure as a suspicious encryption activity by an unknown process.
  • Filter/Exclusion: Create a whitelist exclusion based on the parent process name (MsMpEng.exe, FalconSensor.exe, or ccSvcHst.exe) and restrict the rule to exclude file extensions .zip, .7z, and .gz when accessed by these specific security tools.

• Scenario: Scheduled Backup Jobs with Encryption

  • Context: Automated backup solutions such as Veeam Agent for Microsoft Windows or Acronis Cyber Protect run scheduled tasks that encrypt data before transmission to the cloud or local repository. The rule detects the encryption routine of these agents as a potential “minimum protection” anomaly because they utilize temporary memory buffers similar to ransomware behavior.
  • Filter/Exclusion: Implement an exclusion for processes running under specific Service Accounts (e.g., NT SERVICE\VeeamAgent or AcronisBackupService) and filter out alerts where the process path contains \Program Files\ directories associated with known backup vendors, specifically during defined maintenance windows (e.g., 02:00–04:00).

• Scenario: Software Deployment via Configuration Management

  • Context: Tools like Microsoft System Center Configuration Manager (SCCM) or Ansible Tower frequently deploy encrypted application packages to