Mew501NorthFoxHCC

Hunt Hypothesis

This YARA rule targets specific memory patterns associated with the Mew501NorthFoxHCC signature, indicating the presence of a low-severity malware variant or suspicious code execution. Proactively hunting for this indicator in Azure Sentinel allows the SOC to identify dormant or early-stage intrusions that may evade standard behavioral detections, ensuring timely containment of potential threats.

YARA Rule

rule Mew501NorthFoxHCC
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BE 5B 00 40 00 AD 91 AD 93 53 AD 96 56 5F AC C0 C0 ?? 04 ?? C0 C8 ?? AA E2 F4 C3 00 [2] 00 [3] 00 00 10 40 00 4D 45 57 20 30 2E 31 20 62 79 20 4E 6F 72 74 68 66 6F 78 00 4D 45 57 20 30 2E 31 20 62 79 20 4E 6F 72 74 68 66 6F 78 00 4D 45 57 20 30 2E 31 20 62 79 20 4E 6F 72 74 68 66 6F 78 00 4D 45 57 20 30 2E 31 20 62 79 20 4E 6F 72 74 68 66 6F 78 00 4D }

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

• Scenario: A developer or DevOps engineer runs a local build script that compiles a C/C++ project using gcc or clang with specific optimization flags (e.g., -O2 or -march=native) that result in binary artifacts matching the Mew501NorthFoxHCC signature. This often happens in CI/CD pipelines running on build agents or in developer workstations where intermediate object files or executables are generated in temporary directories.

  • Filter/Exclusion: Exclude paths under standard build directories such as C:\Users\*\AppData\Local\Temp\, C:\builds\, or C:\jenkins\workspace\. Additionally, exclude processes originating from known compiler binaries like gcc.exe, clang.exe, or msbuild.exe if the rule triggers on the creation of new executable files.

• Scenario: An enterprise endpoint protection agent (such as CrowdStrike Falcon, SentinelOne, or Carbon Black) performs a real-time scan or on-demand full-system scan. During this process, the agent may load specific detection modules or generate temporary hash-computation artifacts that inadvertently match the YARA rule’s byte patterns, particularly if the rule targets common header structures or string sequences found in standard PE files.

  • Filter/Exclusion: Exclude processes associated with major EDR solutions, such as FalconSensor.exe, SentinelOneAgent.exe, or cb.exe. You can also exclude file paths located in the vendor’s installation directory, e.g., C:\Program Files\CrowdStrike\ or C:\Program Files\SentinelOne\.

• Scenario: A scheduled maintenance job runs a database backup utility (e.g., SQL Server’s BACKUP command or Oracle’s RMAN) that writes large binary files to a network share or local disk. If the backup file is