HackStop 099 EXE

Hunt Hypothesis

This rule detects the presence of the HackStop 099 executable, a known malware variant often used for initial access or payload delivery. Proactively hunting for this artifact allows the SOC to identify compromised hosts early, as its low severity classification may cause it to be overlooked in standard alerting, enabling faster containment before lateral movement occurs.

YARA Rule

rule HackStop_099_EXE: PEiD
{
    strings:
        $a = { B4 30 CD 21 86 E0 3D 00 03 73 06 B0 FF B4 4C CD 21 50 B8 EB 04 58 EB FB 9A B8 00 33 CD 21 50 E8 D5 03 8D B2 00 B8 01 33 CD 21 50 B8 EB 06 58 EB FB 66 AE 12 E8 C0 03 8D EB 02 40 93 50 55 52 8C D2 FA 50 B8 EB 04 58 EB FB 9A 8C C8 8E D0 EB 02 }
    condition:
        $a 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 system administrator manually downloads and executes the HackStop utility (often used for network troubleshooting or port scanning) from a trusted internal repository or vendor site to diagnose connectivity issues.
  • Filter/Exclusion: Allow execution if the file path matches C:\Tools\HackStop\ or C:\Users\<AdminUser>\Downloads\HackStop* AND the parent process is explorer.exe or cmd.exe initiated by a user in the Domain Admins or Network Ops group.
• Scenario: An automated patching or configuration management script (e.g., Ansible, Puppet, or a custom PowerShell script) copies the HackStop EXE to a target server and executes it as part of a scheduled maintenance window to verify firewall rules or service availability.
  • Filter/Exclusion: Allow execution if the parent process is powershell.exe, python.exe, or ansible-playbook.exe AND the execution time falls within the defined maintenance window (e.g., 02:00–04:00 UTC) AND the user context is a service account (e.g., svc-patching).
• Scenario: A security team performs a periodic vulnerability assessment or penetration test using HackStop as a lightweight scanning tool against non-critical internal assets, running it from a dedicated jump box or analysis workstation.
  • Filter/Exclusion: Allow execution if the source IP address belongs to the Security Team subnet (e.g., 10.20.30.0/24) AND the target machine is in the Test/Dev environment OU AND the user account is in the SecOps group.
• Scenario: A legacy application or custom internal tool bundles the HackStop EXE as a dependency for its own network health check