STNPEE113

Hunt Hypothesis

This detection identifies potential file-based threats by leveraging a specific YARA signature (STNPEE113) to match known malicious patterns within the Azure Sentinel environment. Proactive hunting for this rule is essential to uncover subtle or emerging indicators of compromise that may not trigger high-severity alerts, ensuring early identification of low-fidelity adversary activities before they escalate.

YARA Rule

rule STNPEE113
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 57 56 52 51 53 E8 00 00 00 00 5D 8B D5 81 ED 97 3B 40 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 the false positive scenarios for detection rule STNPEE113, documented with specific enterprise contexts and recommended filters:

• Scenario: Microsoft Office 365 Click-to-Run Self-Update

  • Context: The OfficeC2RClient.exe process frequently spawns child processes to download and install patches for Word, Excel, and PowerPoint. These updates often involve writing new executable files or modifying existing ones in the %ProgramFiles%\Microsoft Office\root\Office16 directory, which can trigger the YARA signature if it scans for new binary creation or specific string patterns within the update payload.
  • Filter/Exclusion: Exclude processes where Process Name equals OfficeC2RClient.exe and Parent Process Name is svchost.exe (or msiexec.exe) running from the path C:\Program Files\Microsoft Office ClickToRun.

• Scenario: Scheduled Antivirus Definition Updates

  • Context: Enterprise endpoints running CrowdStrike Falcon or Symantec Endpoint Protection perform nightly definition updates. During this window, the antivirus engine creates temporary quarantine files and updates its signature database (*.dat or *.cab files) in the installation directory. If STNPEE113 detects specific file hashes or behavioral patterns associated with these update packages, it may flag them as suspicious new artifacts.
  • Filter/Exclusion: Create an exclusion for processes matching FalconSensor.exe (CrowdStrike) or Smc.exe (Symantec) when the File Path contains \ProgramData\Microsoft\Windows Defender\Platform or \Program Files\CrowdStrike\Falcon.

• Scenario: Automated PowerShell Script Execution via Task Scheduler

  • Context: IT Operations teams utilize scheduled tasks to run maintenance scripts (e.g