VxNcuLi1688

Hunt Hypothesis

This hunt hypothesis targets the presence of specific malware signatures identified by the VxNcuLi1688 YARA rule to detect early-stage file-based threats that may evade standard signature updates. Proactively hunting for these indicators in Azure Sentinel allows the SOC team to identify low-severity anomalies before they escalate, ensuring comprehensive coverage against emerging malicious artifacts within the environment.

YARA Rule

rule VxNcuLi1688
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 0E 1E B8 55 AA CD 21 3D 49 4C 74 ?? 0E 0E 1F 07 E8 }

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 VxNcuLi1688 detection rule, including suggested filters and exclusions:

• Scenario: Antivirus Real-Time Scanning of Large Archives

  • Context: Enterprise endpoints running Microsoft Defender for Endpoint or Symantec Endpoint Protection often trigger this rule when scanning large .zip or .tar.gz files containing nested archives. The YARA signature may misinterpret the recursive extraction process as a suspicious memory injection or fileless execution pattern.
  • Filter/Exclusion: Create an exclusion for the specific antivirus engine processes (e.g., MsMpEng.exe, rtvscan64.exe) when they are accessing files within the %TEMP% directory or specific backup folders (e.g., C:\Backup\Daily).

• Scenario: Scheduled PowerShell Script Execution by System Admins

  • Context: A nightly maintenance job triggered by Task Scheduler runs a custom PowerShell script (MaintenanceReport.ps1) that utilizes the .NET Framework to generate logs. The rule flags the temporary compilation of the script into an in-memory assembly as a potential obfuscation technique used by malware.
  • Filter/Exclusion: Exclude alerts where the parent process is TaskScheduler.exe and the command line contains specific keywords like -ExecutionPolicy Bypass or paths matching C:\Scripts\Admin\*.

• Scenario: CI/CD Pipeline Artifact Deployment

  • Context: During a deployment window, Jenkins or GitLab Runner agents on build servers extract application binaries and verify checksums. The rule detects the rapid creation of temporary executable files in the C:\Build\Artifacts directory as suspicious file system activity mimicking dropper behavior.
  • Filter/Exclusion: Implement a path-based exclusion for directories associated with