EXECrypt10ReBirth

Hunt Hypothesis

This detection identifies the execution of the ReBorn variant within the EXECrypt10 family, which is known for encrypting files and displaying ransom notes to disrupt business operations. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to identify early-stage encryption activities before they escalate into full-scale ransomware incidents that could compromise critical data availability.

YARA Rule

rule EXECrypt10ReBirth
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 90 90 60 E8 00 00 00 00 5D 81 ED D1 27 40 00 B9 15 00 00 00 83 C1 04 83 C1 01 EB 05 EB FE 83 C7 56 EB 00 EB 00 83 E9 02 81 C1 78 43 27 65 EB 00 81 C1 10 25 94 00 81 E9 63 85 00 00 B9 96 0C 00 00 90 8D BD 4E 28 40 00 8B F7 AC }

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 EXECrypt10ReBirth detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Scheduled Antivirus Database Updates

  • Context: Enterprise Endpoint Protection suites (e.g., Microsoft Defender for Endpoint or CrowdStrike Falcon) frequently download and install definition updates during off-hours. These processes often spawn temporary executable instances that perform cryptographic operations on the new database files, mimicking the behavior of a “rebirth” or regeneration event detected by the rule.
  • Filter/Exclusion: Create an exclusion based on the process image path containing \Program Files\Microsoft Defender Antivirus\ (or equivalent for CrowdStrike) and restrict the trigger to specific scheduled maintenance windows (e.g., 02:00–04:00 local time).
  • Implementation: Exclude if ProcessImage contains "msmpeng.exe" AND ScheduledJobName = "AntivirusUpdate"

• Scenario: Microsoft Office Click-to-Run Service Updates

  • Context: The Office Click-to-Run service (OfficeClickToRun.exe) performs background updates and integrity checks on the Office installation directory. When it re-indexes or patches core components, it triggers file encryption events that align with the YARA signature’s logic for executable regeneration.
  • Filter/Exclusion: Exclude processes running under the Microsoft Office Click-to-Run Service service account where the parent process is OfficeC2RClient.exe. Additionally, filter out events occurring within the C:\Program Files\Microsoft Office\root\Office16\ directory.
  • Implementation: Exclude if ParentProcessName = "OfficeC2RClient.exe" AND ProcessImage contains "OfficeClickToRun.exe"

• Scenario: Automated Backup and Encryption Jobs (Veeam/Azure)