Human-Machine Interfaces—HMIs—are the operational windows into your control systems. An HMI displays real-time equipment status, production rates, and system alarms. Operators use HMIs to adjust setpoints, manually control equipment, and respond to emergencies. An HMI compromise enables attackers to exfiltrate operational data, inject false information, or issue malicious control commands. HMI hardening is foundational to OT security.
The challenge is that HMIs often run consumer-grade operating systems (Windows, Linux) with weak security posture. They may be connected to corporate networks, internet-accessible, or running legacy software with unpatched vulnerabilities. Hardening requires addressing the operating system, the HMI application, and the network infrastructure.
Operating System Hardening
If your HMI runs Windows, disable unnecessary services, remove network shares not required for operations, and implement Windows hardening per DISA STIGs or CIS Benchmarks. Many organizations configure HMIs as "appliances" with minimal exposed network interfaces and services. Change default passwords and implement role-based login: operators have limited access, administrators require multi-factor authentication.
Patching is critical but operationally complex. Industrial systems often run old versions of Windows or specialized operating systems not actively patched. For systems that cannot be patched, implement compensating controls: network segmentation, intrusion detection, and continuous monitoring to detect exploitation attempts.
HMI Application Security
- Authentication: Require username and password authentication to the HMI. Implement role-based access control: read-only operators cannot issue commands, technicians can control specific equipment, only supervisors can change setpoints.
- Audit Logging: Log all HMI actions: login attempts, data access, control commands, and parameter changes. Include timestamp, user, and what was changed. This creates accountability and enables forensic investigation.
- Configuration Management: HMI configurations (screens, tags, alarms, thresholds) should be version controlled and change-managed. Unexpected configuration changes indicate compromise or unauthorized modification.
- Input Validation: If the HMI allows manual data entry or commands, validate all inputs. Reject malformed commands, out-of-range setpoints, and suspicious patterns. This prevents many command injection attacks.
Network Segmentation and Remote Access
HMIs should operate on a dedicated OT network, isolated from corporate networks and the internet. Use industrial firewalls or switches with VLAN enforcement to separate HMI traffic. If remote access is required—a very common operational need—implement secure remote access: VPN with multi-factor authentication, jump servers that log all remote sessions, and time-limited access (operators cannot maintain persistent remote sessions).
USB ports should be disabled or physically blocked to prevent unauthorized device insertion. HMIs should not have direct internet connectivity. If HMI data must be shared with external systems, use secure data transfer mechanisms (SFTP, API with authentication) rather than direct network access.
HMI hardening is an ongoing process, not a one-time project. We help organizations harden HMIs, implement remote access controls, and establish change management procedures that keep HMIs secure without sacrificing operational efficiency. Contact us to assess your HMI security posture.
This article was written by the Cascadia OT Security practice, which advises Pacific Northwest data centers and manufacturers on industrial cybersecurity. For engagement inquiries, reach our practice team.