ExeJoinerV10Yodaf2f

Hunt Hypothesis

This detection identifies potential file manipulation or execution anomalies flagged by the ExeJoinerV10Yodaf2f YARA signature, which may indicate early-stage adversary activity involving modified executables. Proactively hunting for this behavior in Azure Sentinel is essential to uncover low-severity indicators that could evolve into significant threats before they trigger higher-level alerts.

YARA Rule

rule ExeJoinerV10Yodaf2f
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 68 00 10 40 00 68 04 01 00 00 E8 39 03 00 00 05 00 10 40 00 C6 00 5C 68 04 01 00 00 }

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 ExeJoinerV10Yodaf2f detection rule, including targeted filters and exclusions:

• Scenario: Microsoft Office Click-to-Run Service Updates

  • Context: The OfficeClickToRun.exe process frequently spawns child processes to merge or update application components during background maintenance windows. This behavior mimics the “joining” of executable modules detected by the YARA rule.
  • Filter/Exclusion: Create a whitelist exclusion for the parent process path: C:\Program Files\Microsoft Office\root\Office16\ClickToRun.exe. Additionally, exclude any child processes spawned with command lines containing --update or --merge.

• Scenario: Antivirus Real-Time Scanning (CrowdStrike/Falcon)

  • Context: Endpoint Detection and Response (EDR) agents like CrowdStrike Falcon or Microsoft Defender often utilize a “joiner” utility to bundle threat definitions or scan artifacts. The EDR agent’s internal service (C:\Program Files\CrowdStrike\csagent.exe) may trigger this rule when performing definition updates.
  • Filter/Exclusion: Exclude all processes where the ImageName matches known EDR agents (e.g., csagent.exe, MsMpEng.exe, falcon-sensor.exe). Add a condition to ignore events where the process is running under the SYSTEM or NT AUTHORITY\LOCAL SERVICE user context.

• Scenario: Scheduled PowerShell Deployment Jobs

  • Context: IT Operations teams often run scheduled tasks using powershell.exe to deploy patches or configuration scripts that invoke custom joiner utilities (e.g., ExeJoiner.dll loaded via reflection). These jobs typically run during off-hours under a dedicated service account.
  • **Filter/Exclusion