RJoiner12byVaska250320071658

Hunt Hypothesis

This YARA rule targets specific memory patterns or code artifacts associated with the RJoiner12byVaska250320071658 signature, potentially indicating the presence of a low-severity custom implant or obfuscated payload in process memory. Proactively hunting for this signature allows the SOC team to identify stealthy, low-fidelity adversary footholds that may evade traditional network-based detections before they are leveraged for lateral movement or data exfiltration.

YARA Rule

rule RJoiner12byVaska250320071658
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 81 EC 0C 02 00 00 8D 85 F4 FD FF FF 56 50 68 04 01 00 00 FF 15 14 10 40 00 90 8D 85 F4 FD FF FF 50 FF 15 10 10 40 00 90 BE 00 20 40 00 90 83 3E FF 0F 84 84 00 00 00 53 57 33 FF 8D 46 }

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 operations engineer uses a custom Python or PowerShell script to perform bulk string concatenation or log parsing on large datasets, where the script explicitly imports or references a library/module named RJoiner12 (or similar) for performance optimization.
  • Filter/Exclusion: Exclude processes where the parent process is python.exe, pwsh.exe, or powershell.exe and the command line contains arguments like parse, concat, or log_analysis. Alternatively, whitelist the specific script path (e.g., C:\Scripts\utils\joiner.py).
• Scenario: A CI/CD pipeline agent (e.g., Jenkins, GitLab Runner, or Azure DevOps Agent) executes a build step that invokes a custom build tool or plugin named RJoiner12byVaska250320071658 (or a binary with this name) to merge artifact files or generate release notes.
  • Filter/Exclusion: Exclude processes where the parent process is a known CI/CD agent (e.g., jenkins-agent.exe, gitlab-runner.exe, vsts-agent.exe) and the working directory is within the standard build workspace (e.g., C:\Jenkins\workspace\, C:\gitlab-runner\builds\).
• Scenario: A database administrator runs a maintenance script that uses a custom utility RJoiner12 to join large CSV or TSV files before loading them into a data warehouse, often triggered by a scheduled task during off-peak hours.
  • Filter/Exclusion: Exclude processes where the parent process is cmd.exe or powershell.exe and the command line contains keywords like db_maint, load, or warehouse, and the user account is a service account (e.g., `svc