Regla para detectar Ransom.Petya con md5 AF2379CC4D607A45AC44D62135FB7015

Hunt Hypothesis

This hunt hypothesis aims to identify early-stage Ransom.Petya activity by monitoring for the execution of a specific malicious file signature (MD5: AF2379CC4D607A45AC44D62135FB7015) across Azure Sentinel endpoints. Proactively hunting for this indicator is critical because, despite its low severity classification, detecting this known ransomware variant before widespread encryption allows the SOC team to isolate affected assets and prevent potential data loss or operational downtime.

YARA Rule

rule Ransom_Petya {
meta:
    description = "Regla para detectar Ransom.Petya con md5 AF2379CC4D607A45AC44D62135FB7015"
    author = "CCN-CERT"
    version = "1.0"
strings:
    $a1 = { C1 C8 14 2B F0 03 F0 2B F0 03 F0 C1 C0 14 03 C2 }
    $a2 = { 46 F7 D8 81 EA 5A 93 F0 12 F7 DF C1 CB 10 81 F6 }
    $a3 = { 0C 88 B9 07 87 C6 C1 C3 01 03 C5 48 81 C3 A3 01 00 00 }
condition:
    all of them
}

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 3 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the Ransom.Petya detection rule (MD5: AF2379CC4D607A45AC44D62135FB7015), along with suggested filters or exclusions tailored for an enterprise environment:

• Enterprise Backup Software Scanning Legacy Archives

  • Scenario: The MD5 AF2379CC4D607A45AC44D62135FB7015 corresponds to a known, legitimate version of the Petya executable often bundled within legacy backup agents or specific patch management tools. In this scenario, Veeam Backup & Replication or Commvault is performing its scheduled nightly scan, accessing and temporarily locking this binary on file servers to verify integrity before archiving old data sets.
  • Filter/Exclusion: Create a rule exclusion for the process name vbr.exe (Veeam) or SimAgent.exe (Commvault) when the parent process is running under the SYSTEM account, specifically targeting file paths within the backup repository directories (e.g., C:\Program Files\Veeam\Backup\).

• Scheduled Antivirus Definition Updates

  • Scenario: The organization utilizes a third-party Endpoint Detection and Response (EDR) solution that periodically downloads and validates signature databases. During the weekly update cycle, tools like CrowdStrike Falcon or SentinelOne extract and verify the Petya detection module (matching the specific MD5) as part of their internal self-check routine before pushing new definitions to endpoints.
  • Filter/Exclusion: Exclude alerts where the initiating process belongs to the EDR vendor’s service group (e.g., C:\Program Files\CrowdStrike\cs_falcon.exe or