FSGv20bartxt

Hunt Hypothesis

This detection identifies potential file-based threats or specific malware signatures matching the “FSGv20bartxt” YARA pattern within Azure Sentinel’s ingested telemetry. Proactive hunting for this indicator is essential to uncover early-stage compromises that may evade standard signature-based alerts, allowing the SOC team to investigate lateral movement or data exfiltration before an incident escalates.

YARA Rule

rule FSGv20bartxt
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 87 25 [3] 00 61 94 55 A4 B6 80 FF 13 }

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 FSGv20bartxt detection rule in an enterprise environment, along with recommended filters and exclusions:

• Scenario: Automated Backup Script Execution

  • Context: The nightly backup job executed by Veeam Backup & Replication or Commvault generates temporary .txt log files containing binary-like structures that match the FSGv20bartxt signature. These files are created in the C:\ProgramData\Veeam\Backup\Logs directory during the backup window.
  • Filter/Exclusion: Create a rule exclusion based on the Process Name (Veeam.Backup.Service.exe) and File Path (*\\Veeam\\Backup\\Logs\\*.txt). Alternatively, add these specific paths to the YARA rule’s exclude section.

• Scenario: Endpoint Protection Health Checks

  • Context: The Microsoft Defender for Endpoint (or CrowdStrike Falcon) agent performs a scheduled “Health Check” task every 4 hours. This task generates a diagnostic text report (health_report.txt) in the %ProgramFiles%\Windows Defender directory which contains specific byte patterns triggering the rule.
  • Filter/Exclusion: Implement an exclusion for processes running under the Microsoft Windows user account (e.g., SYSTEM or NT AUTHORITY\SYSTEM) where the executable path matches *\\Windows Defender\\*.exe.

• Scenario: IT Asset Management Inventory Scans

  • Context: The ServiceNow Discovery or Landesk agent runs a scheduled inventory scan on workstations. During this process, it writes a temporary text manifest (inventory_manifest.txt) to the C:\Temp directory containing structured data that mimics the detection logic of FSGv20bartxt.