BJFntv12RC

Hunt Hypothesis

This YARA rule targets specific binary patterns or code structures that may indicate the presence of a low-severity threat actor implant or a known malware variant within the environment. Proactively hunting for these signatures in Azure Sentinel allows the SOC team to identify dormant or stealthy footholds before they escalate into more impactful post-exploitation activities or lateral movement.

YARA Rule

rule BJFntv12RC
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 02 69 B1 83 EC 04 EB 03 CD 20 EB EB 01 EB 9C EB 01 EB EB }

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

• Legacy .NET Framework Compilation Artifacts: Developers or CI/CD pipelines using older versions of MSBuild or Visual Studio (pre-.NET Core) may leave intermediate compilation objects or specific debug symbols in build directories that match the byte patterns of BJFntv12RC. These are often found in obj/ or bin/Debug folders.
  • Filter/Exclusion: Exclude file paths containing \obj\, \bin\, or \build\ directories, and restrict the rule to executable extensions (.exe, .dll) rather than all file types.
• Third-Party .NET Library Dependencies: Common enterprise libraries (e.g., specific versions of Newtonsoft.Json, Entity Framework, or custom internal SDKs) may contain embedded metadata or resource strings that coincidentally match the YARA signature. This is frequent in large monolithic applications where dependencies are bundled into a single assembly.
  • Filter/Exclusion: Create a whitelist of known-good hashes (SHA256) for critical third-party DLLs used in the environment, or exclude files located in standard dependency folders like \packages\ or \lib\.
• Scheduled Backup or Archive Jobs: Enterprise backup solutions (e.g., Veeam, Commvault) or log rotation scripts that compress or archive application binaries may create temporary copies or split archives that retain the original file’s internal structure. If the YARA rule scans raw bytes without accounting for compression headers, these temporary files can trigger alerts.
  • Filter/Exclusion: Exclude files with common archive extensions (.zip, .tar, .gz, .7z) and files located in designated backup staging areas (e.g., \Backup\, \Temp\Backup\).
• Development Environment IDE Caches: Integrated Development Environments (IDEs) like Visual Studio or JetBrains Rider maintain local caches of compiled assemblies and project files