EXEStealthv271

Hunt Hypothesis

This hunt hypothesis targets adversaries utilizing custom or obfuscated executable files that evade standard signature-based detection by matching specific YARA patterns associated with the EXEStealthv271 rule. Proactive hunting in Azure Sentinel is essential to identify these stealthy binaries early, as their low severity classification suggests they may be missed by automated alerts yet represent a critical initial foothold for advanced persistent threats.

YARA Rule

rule EXEStealthv271
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 00 60 EB 00 E8 00 00 00 00 5D 81 ED B0 27 40 }

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 5 specific false positive scenarios for the EXEStealthv271 detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Office Click-to-Run Updates

  • Context: The OfficeClickToRun.exe process frequently spawns child processes in temporary directories (%TEMP%) to apply minor updates or repair components. These often involve extracting executable payloads that match the “stealth” signature of EXEStealthv271 due to their dynamic loading behavior.
  • Filter/Exclusion: Exclude processes where ImageName contains OfficeClickToRun.exe AND ParentImageName is Microsoft Office Click-to-Run Service. Additionally, filter out alerts where the file path starts with C:\Program Files\Microsoft Office Root\Office16\ or %TEMP%\OfficeUpdates\.

• Scenario: Antivirus Heuristic Scanning (CrowdStrike/Defender)

  • Context: Endpoint Detection and Response (EDR) agents like CrowdStrike Falcon or Microsoft Defender often spawn temporary scanning executables that inject code into other processes to perform real-time protection. This injection mimics the stealthy execution patterns detected by EXEStealthv271, particularly when scanning large archives in background queues.
  • Filter/Exclusion: Create an exclusion for parent processes named CnsSys.exe (CrowdStrike) and MsMpEng.exe (Defender). Filter alerts where the CommandLine contains keywords like /scan, /heuristic, or /background.

• Scenario: Scheduled PowerShell Deployment Jobs

  • Context: IT administrators often use scheduled tasks to deploy configuration scripts via PowerShell.exe. These jobs frequently invoke temporary .exe wrappers (e.g., from SCCM or Intune) that execute silently in the background. The rule may flag