ExeStealth275aWebtoolMaster

Hunt Hypothesis

This detection identifies potential web tool execution anomalies that may indicate early-stage reconnaissance or lateral movement by adversaries utilizing custom executable signatures. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to uncover stealthy threats that might evade standard signature-based defenses and refine their YARA rules based on observed patterns.

YARA Rule

rule ExeStealth275aWebtoolMaster
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 58 53 68 61 72 65 77 61 72 65 2D 56 65 72 73 69 6F 6E 20 45 78 65 53 74 65 61 6C 74 68 2C 20 63 6F 6E 74 61 63 74 20 73 75 70 70 6F 72 74 40 77 65 62 74 6F 6F 6C 6D 61 73 74 65 72 2E 63 6F 6D 20 2D 20 77 77 77 2E 77 65 62 74 6F 6F 6C 6D 61 73 74 65 72 }

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 3-5 specific false positive scenarios for the ExeStealth275aWebtoolMaster detection rule, including suggested filters and exclusions:

• Scenario: Automated Web Application Firewall (WAF) Health Checks

  • Context: The organization’s WAF (e.g., F5 BIG-IP or NGINX Plus) runs a scheduled health check script every 5 minutes that executes the ExeStealth275aWebtoolMaster binary to probe endpoint connectivity and verify SSL certificate validity. This triggers the rule due to the tool’s network scanning behavior, which mimics reconnaissance activity often associated with stealth tools.
  • Suggested Filter: Create an exclusion based on the specific service account used by the WAF (e.g., svc-waf-healthcheck) or exclude traffic originating from the dedicated DMZ subnet (10.20.40.0/24) where these probes originate.

• Scenario: Patch Management Deployment via SCCM

  • Context: During the nightly maintenance window, Microsoft Endpoint Configuration Manager (SCCM) deploys a silent update to the ExeStealth275aWebtoolMaster application on 500+ workstations simultaneously. The installation process involves the tool launching in the background to configure new registry keys and network policies, generating file system and process creation events that match the YARA signature for “webtool master” initialization.
  • Suggested Filter: Exclude alerts where the parent process is ccmexec.exe (SCCM agent) or filter by time window (e.g., 02:00–04:00 UTC) to align with the known maintenance schedule.

• Scenario: Enterprise Browser Extension Auto-Update

  • Context: The IT department enforces a custom Chrome/Edge extension for secure