Regla para detectar W32/Filecoder.Alfa (Posibles falsos positivos)

Hunt Hypothesis

This hunt hypothesis targets adversary behavior involving the execution of the W32/Filecoder.Alfa ransomware variant to identify potential encryption activities that may initially trigger false positives. Proactive hunting in Azure Sentinel is essential to distinguish genuine early-stage ransomware infections from benign noise, ensuring rapid response before data exfiltration or widespread system locking occurs.

YARA Rule

rule Ransom_Alfa
{
meta:
description = "Regla para detectar W32/Filecoder.Alfa (Posibles falsos positivos)"
author = "CCN-CERT"
version = "1.0"
strings:
$a = { 8B 0C 97 81 E1 FF FF 00 00 81 F9 19 04 00 00 74 0F 81 F9 } 
$b = { 22 04 00 00 74 07 42 3B D0 7C E2 EB 02 }
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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific scenarios that could trigger the W32/Filecoder.Alfa detection rule in a legitimate enterprise environment, along with suggested filters or exclusions to reduce false positives:

• Enterprise Backup and Snapshot Operations

  • Scenario: Scheduled backup agents (e.g., Veeam Agent, Commvault, or Microsoft System Center Data Protection Manager) performing incremental backups often lock files, read metadata, and create temporary shadow copies. The rapid creation of new file handles and the modification of file attributes during these snapshots can mimic the “file enumeration” behavior typical of ransomware.
  • Filter/Exclusion: Create an exclusion for the specific service accounts running the backup software (e.g., NT SERVICE\VeeamBackupService) or exclude the known executable paths (C:\Program Files\Veeam\...). Additionally, filter out events where the parent process is a known backup utility and the action involves file locking rather than mass encryption.

• Large-Scale Software Deployment via SCCM/Intune

  • Scenario: During off-hours maintenance windows, deployment tools like Microsoft Endpoint Configuration Manager (SCCM) or Microsoft Intune push updates to thousands of endpoints simultaneously. These agents often extract large installation packages (MSI/EXE) to temporary directories (C:\Windows\CCMCache), creating and modifying hundreds of files in rapid succession, which triggers the “mass file modification” logic of the Filecoder rule.
  • Filter/Exclusion: Exclude processes originating from the SCCM client agent (ccmexec.exe) or Intune Management Extension (IntuneManagementExtension). Implement a filter that ignores activity within specific deployment directories (e.g., C:\Windows\CCMCache, C:\Program Files\Microsoft Intune Management Extension) during known maintenance windows.

• **Antivirus Real-Time Scanning and Quarantine