DEFv100Engbartxt

Hunt Hypothesis

This hunt hypothesis targets adversaries leveraging specific English-language text patterns identified by the DEFv100Engbartxt YARA signature to detect potential data exfiltration or command-and-control communication within Azure Sentinel. Proactive hunting for this low-severity indicator is essential to uncover subtle, early-stage threats that may evade standard alerting thresholds and prevent lateral movement before escalation occurs.

YARA Rule

rule DEFv100Engbartxt
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BE ?? 01 40 00 6A ?? 59 80 7E 07 00 74 11 8B 46 0C 05 00 00 40 00 8B 56 10 30 10 40 4A 75 FA 83 C6 28 E2 E4 68 [2] 40 00 C3 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 }

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 3-5 specific false positive scenarios for the DEFv100Engbartxt detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Defender Antivirus Signature Updates

  • Context: The YARA rule may flag the MpCmdG.exe process or associated update files when Microsoft Defender downloads new engine definitions. These updates often contain compressed binary blobs that match the “Engbartxt” signature pattern due to similar header structures or embedded text strings.
  • Suggested Filter/Exclusion: Create an exclusion for the file path C:\Program Files\Windows Defender\MpCmdG.exe and exclude any process tree initiated by MsMpSvc service on Windows Server 2016+ machines.

• Scenario: Scheduled PowerShell Script Execution via Task Scheduler

  • Context: An enterprise-wide automated maintenance script (e.g., a nightly log rotation or inventory check) runs as a scheduled task using powershell.exe. If the script dynamically generates temporary text files or reads configuration manifests containing specific English text patterns, it triggers the rule. This is common in environments using tools like SCCM or Ansible.
  • Suggested Filter/Exclusion: Exclude processes where the parent process is svchost.exe (specifically the Schedule service) and the command line contains keywords such as “TaskScheduler” or specific script names like InventoryCheck.ps1.

• Scenario: Third-Party Backup Agent File Scanning

  • Context: Enterprise backup solutions like Veeam, Rubrik, or Commvault often spawn temporary worker processes that read and write large text-based manifest files during the backup window. The YARA rule might misinterpret these manifest headers as suspicious activity, especially if the agent is running on a high-I/O storage server.