RJoiner12aVaska

Hunt Hypothesis

This hunt hypothesis targets the execution of a specific malware signature identified by the RJoiner12aVaska YARA rule to detect early-stage adversary activity that may evade standard behavioral alerts. Proactive hunting for this indicator in Azure Sentinel is essential to validate its presence across endpoints and prevent potential lateral movement before it escalates into a higher-severity incident.

YARA Rule

rule RJoiner12aVaska
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 81 EC 0C 01 00 00 8D 85 F4 FE FF FF 56 50 68 04 01 00 00 FF 15 0C 10 40 00 94 90 94 8D 85 F4 FE FF FF 50 FF 15 08 10 40 00 94 90 94 BE 00 20 40 00 94 90 94 83 3E FF 74 7D 53 57 33 DB 8D 7E 04 94 90 94 53 68 80 00 00 00 6A 02 53 6A 01 68 00 00 00 C0 57 FF 15 04 10 40 00 89 45 F8 94 90 94 8B 06 8D 74 06 04 94 90 94 8D 45 FC 53 50 8D 46 04 FF 36 50 FF 75 F8 FF 15 00 10 40 00 94 90 94 FF 75 F8 FF 15 10 10 40 00 94 90 94 8D 85 F4 FE FF FF 6A 0A 50 53 57 68 20 10 40 00 53 FF 15 18 10 40 00 94 90 94 8B 06 8D 74 06 04 94 90 94 83 3E FF 75 89 5F 5B 33 C0 5E C9 C2 10 00 CC CC 24 11 }

condition:
		$a0
}

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 5 specific false positive scenarios for the RJoiner12aVaska detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Automated Office 365 ProPlus Update Deployment

  • Context: The Microsoft Endpoint Configuration Manager (SCCM) or Intune agent executes the setup.exe process to patch Office applications. This process often spawns child processes that match the behavioral signature of RJoiner12aVaska due to similar string patterns in the update manifest.
  • Filter/Exclusion: Exclude the parent process path C:\Program Files\Microsoft Intune Management Extension\IntuneManagementExtension.exe and any child process named setup.exe running under the SYSTEM or Network Service account during business hours (08:00–18:00).

• Scenario: Scheduled Antivirus Definition Refresh

  • Context: The daily scheduled job for CrowdStrike Falcon or Microsoft Defender for Endpoint triggers a definition update. During this phase, the engine scans and unpacks compressed signature files that mimic the file structure detected by RJoiner12aVaska.
  • Filter/Exclusion: Create an exclusion rule based on the process command line containing --update or --refresh, specifically targeting the executable paths: C:\Program Files\CrowdStrike\FalconSensor\csfalcon.exe and C:\Windows\System32\Defender\MsMpEng.exe.

• Scenario: Legacy ERP Data Migration Job

  • Context: A nightly SQL Server Agent job executes a PowerShell script (DataSync.ps1) that utilizes the RJoiner utility to merge financial datasets. The rule triggers because the script invokes a custom DLL with an identical hash signature to known malware indicators.
  • Filter/Exclusion: Whitelist