BJFntv11b

Hunt Hypothesis

This detection identifies potential malicious file execution patterns captured by the BJFntv11b YARA signature, which may indicate early-stage adversary activity involving specific software artifacts or known malware families. A proactive hunt is essential within Azure Sentinel to validate these low-severity alerts against broader telemetry, ensuring that subtle indicators of compromise are not overlooked before they escalate into significant incidents.

YARA Rule

rule BJFntv11b
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 01 EA 9C EB 01 EA 53 EB 01 EA 51 EB 01 EA 52 EB 01 EA 56 }

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 BJFntv11b detection rule, including suggested filters and exclusions:

• Scenario: Automated Antivirus Definition Updates

  • Context: The YARA rule triggers when the enterprise endpoint protection suite (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint, or Symantec Endpoint Protection) downloads and installs new virus definition signatures. These updates often involve the creation of temporary executable files in specific directories that match the heuristic patterns defined in BJFntv11b.
  • Suggested Filter/Exclusion: Exclude file paths containing standard update directories, such as C:\ProgramData\McAfee\Agent\Updates or C:\Windows\System32\catroot2, and filter out processes where the parent process is the specific AV service (e.g., falcon.sys or MsMpEng.exe).

• Scenario: Scheduled PowerShell Script Execution for Patch Management

  • Context: During nightly maintenance windows, IT administrators run scheduled tasks using Microsoft SCCM or Ansible Tower that execute custom PowerShell scripts to deploy patches. These scripts often generate temporary .ps1 files and invoke powershell.exe with arguments that mimic the behavior detected by the rule.
  • Suggested Filter/Exclusion: Create an exclusion for processes where the command line contains keywords like /scheduled, -File, or specific script paths (e.g., C:\Scripts\PatchManagement\Deploy.ps1). Additionally, exclude events occurring within a defined time window (e.g., 02:00–04:00 UTC) if the rule is known to trigger heavily during maintenance hours.

• Scenario: Software Deployment via Configuration Management Tools

  • Context: When Jenkins or Azure DevOps pipelines deploy new applications