FileShield

Hunt Hypothesis

This rule identifies files that have been modified or protected by FileShield, a mechanism often used by adversaries to obscure malicious payloads or prevent analysis through file locking and attribute manipulation. Proactively hunting for these indicators in Azure Sentinel allows the SOC to detect potential living-off-the-land techniques or early-stage persistence mechanisms before they are leveraged for lateral movement or privilege escalation.

YARA Rule

rule FileShield: Packer PEiD
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 50 1E EB ?? 90 00 00 8B D8 }

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

• Antivirus/EDR Real-Time Scanning: Security agents like Microsoft Defender for Endpoint, CrowdStrike Falcon, or SentinelOne often perform in-memory or on-disk scanning of executable files. If the YARA rule targets specific file headers, entropy values, or known binary patterns, these agents may temporarily lock or read the file in a way that triggers the detection logic.

  • Filter/Exclusion: Exclude processes matching MsMpEng.exe (Defender), FalconSensor.exe (CrowdStrike), or SOneAgent.exe (SentinelOne) as the parent process, or exclude file paths located in standard AV cache directories (e.g., C:\ProgramData\Microsoft\Windows Defender\).

• Software Deployment and Patching: Enterprise IT teams use tools like SCCM (System Center Configuration Manager), PDQ Deploy, or Ansible to push software updates or install new applications. These tools frequently write, verify, and execute installer packages (.msi, .exe) which may match generic file structure signatures or specific version strings targeted by the rule.

  • Filter/Exclusion: Exclude file paths under C:\Windows\Installer\, C:\Program Files\, or C:\Program Files (x86)\ when the parent process is ccmexec.exe (SCCM), PDQDeploy.exe, or ansible-playbook.

• Backup and Archiving Agents: Backup solutions such as Veeam, Commvault, or Acronis create temporary copies or snapshots of data during backup windows. If the rule detects specific file types (e.g., .zip, .7z, or database files) based on content or naming conventions, these agents may trigger alerts when accessing or writing to backup repositories.

  • Filter/Exclusion: Exclude processes named vbr.exe (Veeam Backup & Rep