FACRYPTv10

Hunt Hypothesis

This detection identifies potential file encryption activities characteristic of ransomware or data exfiltration attempts by analyzing specific YARA signatures within Azure Sentinel workloads. Proactively hunting for these patterns allows the SOC team to uncover early-stage adversary behaviors that may not trigger high-severity alerts, enabling faster containment before widespread system impact occurs.

YARA Rule

rule FACRYPTv10
{
      meta:
		author="malware-lu"
strings:
		$a0 = { B9 [2] B3 ?? 33 D2 BE [2] 8B FE AC 32 C3 AA 49 43 32 E4 03 D0 E3 }

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 FACRYPTv10 detection rule, including recommended filters and exclusions tailored for an enterprise environment:

• Scenario: Automated Backup Encryption by Veeam or Commvault

  • Context: Enterprise backup agents (e.g., Veeam Backup & Replication, Commvault) frequently spawn child processes to encrypt large data sets before transmission. These processes often utilize standard cryptographic libraries that match the YARA signatures for fileless encryption behaviors.
  • Filter/Exclusion: Exclude processes where the parent process is VeeamAgent.exe or commserve.exe, and the executable path resides within the designated backup installation directory (e.g., C:\Program Files\Veeam\Backup and Replication\).

• Scenario: Scheduled Office 365 ProPlus Update & Configuration Tasks

  • Context: The Microsoft Office Click-to-Run service (OfficeClickToRun.exe) or the scheduled task “Microsoft Office Background Task” often performs local configuration updates that involve encrypting user preference files and registry keys, triggering the rule during non-business hours.
  • Filter/Exclusion: Create an exclusion for officeclicktorun.exe running under the context of the SYSTEM account or specific scheduled tasks named “OfficeBackgroundTask” located in C:\Program Files\Microsoft Office\root\Office16.

• Scenario: Endpoint DLP Agent Scanning (e.g., Symantec DLP or Forcepoint)

  • Context: Data Loss Prevention agents actively scan and encrypt sensitive documents on the endpoint before allowing network transmission. The encryption engine within these agents often mimics the behavior of a cryptominer or ransomware, causing high CPU usage and file locking events that trigger FACRYPTv10.
  • Filter/Exclusion: Exclude processes with names containing