MEW510Northfox

Hunt Hypothesis

This detection identifies potential Northfox adversary activity by leveraging a specific YARA signature to catch known malware or file artifacts associated with their campaigns. SOC teams should proactively hunt for this indicator in Azure Sentinel to validate early-stage compromises and correlate low-severity alerts with broader threat intelligence before adversaries establish persistence.

YARA Rule

rule MEW510Northfox
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BE 5B 00 40 00 AD 91 AD 93 53 AD 96 56 5F AC C0 C0 }

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 MEW510Northfox detection rule, including suggested filters and exclusions:

• Scenario: Antivirus Engine Self-Scanning

  • Context: The YARA rule may flag the scanning process of enterprise endpoint protection suites (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint, or SentinelOne) when they perform real-time file integrity checks on their own installation directories. This often triggers alerts due to high-frequency memory access and signature matching against known binaries.
  • Suggested Filter/Exclusion: Create an exclusion rule based on the process image path and parent process name. Exclude any detection where ProcessImage contains \Program Files\CrowdStrike\ or \Windows\System32\wdigest.exe AND ParentProcessName is csfalcon.exe, MsMpEng.exe, or SentinelOneAgent.exe.

• Scenario: Scheduled Backup Agent Execution

  • Context: Enterprise backup solutions like Veeam Backup & Replication, Rubrik, or Commvault often run scheduled jobs during off-hours that involve heavy file I/O and temporary file creation. The YARA signature might misinterpret the rapid generation of temporary metadata files or log rotation by these agents as suspicious activity indicative of a new malware strain.
  • Suggested Filter/Exclusion: Implement a time-based exclusion combined with process whitelisting. Exclude alerts where ProcessName matches vrb.exe (Veeam), rbd_service.exe (Rubrik), or cmdagent.exe (Commvault) occurring between 01:00 and 05:00 local server time.

• Scenario: Software Deployment via Configuration Management

  • Context: Automated deployment tools such as **Microsoft Endpoint Configuration Manager (ME