EXEStealth275WebtoolMaster

Hunt Hypothesis

This YARA rule identifies the presence of the EXEStealth275WebtoolMaster executable, a known web tool often utilized by adversaries for initial access or lateral movement. Proactively hunting for this artifact in Azure Sentinel allows the SOC team to detect low-severity footholds that may have been missed by standard behavioral detections, enabling early containment before the tool is leveraged for further compromise.

YARA Rule

rule EXEStealth275WebtoolMaster
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 90 60 90 E8 00 00 00 00 5D 81 ED D1 27 40 00 B9 15 00 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

• Scenario: A developer or DevOps engineer uses Visual Studio Code or JetBrains Rider to debug a web application locally, where the IDE spawns a helper executable (e.g., node.exe, dotnet.exe, or a specific debugger binary) that matches the “Webtool” signature due to its embedded JavaScript engine or HTTP client libraries.

  • Filter/Exclusion: Exclude processes where the parent process is a known IDE (e.g., Code.exe, idea64.exe, rider64.exe) or where the executable path resides within standard development directories (e.g., C:\Users\<User>\AppData\Local\Programs\Microsoft VS Code\ or C:\Program Files\JetBrains\).

• Scenario: An automated CI/CD pipeline agent (such as Jenkins, GitLab Runner, or Azure DevOps Agent) executes a build step that compiles or runs a lightweight web-based test harness or mock server (e.g., a custom test-server.exe or mock-api.exe) that bundles web tooling libraries, triggering the YARA rule during routine nightly builds.

  • Filter/Exclusion: Exclude processes where the parent process is a known CI/CD agent (e.g., jenkins.exe, gitlab-runner.exe, vsts-agent.exe) or where the working directory contains CI/CD specific artifacts (e.g., \.jenkins\, \.gitlab\runner\, \.azure-devops\).

• Scenario: A system administrator runs a scheduled Windows Task to execute a custom PowerShell script that launches a local web-based dashboard or monitoring tool (e.g., Grafana.exe, InfluxDB.exe, or a custom metrics-viewer.exe) for internal status checks, where the executable is signed but still matches the generic ”