NETDLLMicrosoft

Hunt Hypothesis

This detection identifies potential code injection or DLL hijacking attempts by monitoring for Microsoft .NET assemblies that match specific YARA signatures within the Azure Sentinel environment. Proactive hunting is essential to uncover stealthy lateral movement or persistence mechanisms where adversaries leverage legitimate .NET components to execute malicious payloads that may evade standard signature-based defenses.

YARA Rule

rule NETDLLMicrosoft
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 00 00 00 00 00 00 00 00 5F 43 6F 72 44 6C 6C 4D 61 69 6E 00 6D 73 63 6F 72 65 65 2E 64 6C 6C 00 00 ?? 00 00 FF 25 }

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 4 specific false positive scenarios for the NETDLLMicrosoft detection rule in an enterprise environment, including suggested filters or exclusions:

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

  • Context: The OfficeClickToRun.exe process (part of the Microsoft Office suite) frequently loads internal .NET DLLs during background update checks and feature installations. This often triggers the rule when the service runs via the Windows Task Scheduler or the Update Orchestrator.
  • Filter/Exclusion: Exclude processes where ImageName equals OfficeClickToRun.exe AND CommandLine contains /update. Alternatively, add a path exclusion for C:\Program Files\Microsoft Office\root\Office16\*.

• Scenario: Azure Arc or Microsoft Endpoint Manager (Intune) Agent Activity

  • Context: The Intune Management Extension (IntuneManagementExtension.exe) and the Azure Connected Machine agent often execute .NET-based scripts to inventory hardware, apply policies, or push configurations. These agents load standard Microsoft .NET DLLs that match the YARA signature during their scheduled maintenance windows (typically early morning).
  • Filter/Exclusion: Create an exclusion for ImageName containing IntuneManagementExtension.exe, Microsoft.Sense.Service.Host.exe, and AzureConnectedMachineAgent.exe. Ensure the filter applies specifically to processes running under the SYSTEM or NT SERVICE\IntuneManagementExtension user context.

• Scenario: Scheduled PowerShell Health Checks via Task Scheduler

  • Context: Enterprise IT teams often deploy scheduled tasks (e.g., “Daily Disk Cleanup” or “License Audit”) that execute .ps1 scripts using the powershell.exe host. These scripts utilize Microsoft-provided .NET assemblies for reporting, causing the YARA rule to fire on the parent process when it loads these