Why Legacy Equipment Creates a Special Challenge

Many industrial facilities operate equipment that has remained in service for years or even decades. Older legacy PLCs, HMIs, engineering workstations, and communication devices may still perform their original functions reliably.

Replacing these systems can be expensive and disruptive. However, connecting older equipment to modern monitoring, analytics, and enterprise applications can introduce new cybersecurity risks.

Legacy systems may lack modern authentication, encryption, logging, or security update mechanisms. Some devices also depend on communication protocols that were designed for trusted local networks rather than exposure to broader environments.

Recent industrial cybersecurity discussions increasingly emphasize the need to secure older assets as manufacturers expand OT connectivity.

Start with Asset Visibility

The first step is understanding what is connected. Create an inventory of PLCs, touchscreen HMIs, engineering workstations, industrial switches, gateways, remote access devices, and other network-connected assets.

Record manufacturer, model, firmware, software version, network address, communication role, physical location, and responsible maintenance team where possible.

Do not rely on an automated network scan alone. Some older devices may respond poorly to unexpected traffic. Use approved discovery methods and coordinate with control-system engineers before performing active tests on production networks.

Segment the Industrial Network

Network segmentation limits unnecessary communication between different areas of an industrial environment. For example, controllers, operator stations, engineering workstations, and enterprise applications may require different access policies.

Use documented network zones and controlled communication paths. Firewalls, Ethernet communication interfaces, or industrial security gateways can restrict traffic to approved destinations, services, and protocols.

Segmentation should reflect the actual process architecture. A poorly designed firewall rule can block essential control communication or create an unsafe operating condition. Test changes in a controlled manner and maintain a rollback plan.

Use Gateways Carefully

Industrial gateways and specialized communication modules can provide a controlled interface between legacy devices and newer applications. They may translate protocols, collect data, or provide a defined communication boundary.

However, a gateway is not automatically a security solution. It must be configured with appropriate access controls, communication restrictions, firmware maintenance, and monitoring.

Where possible, use read-only data collection for monitoring applications. If an application needs to send commands toward a controller, define exactly which commands are permitted, who can authorize them, and how the operation is recorded.

Manage Remote Access

Remote access can help equipment suppliers and maintenance teams diagnose problems without traveling to a site. It also creates an entry point that requires careful management.

Disable unused remote access services. Require approved authentication methods, restrict access to specific assets, and provide time-limited authorization where appropriate.

Remote sessions should be logged and monitored. Shared accounts and permanent unrestricted connections make it difficult to determine who accessed a system and what changes were made.

Where practical, require an authorized site representative to approve remote maintenance sessions and supervise actions that may affect production.

Plan Patching Around Availability

Older OT devices may not support current operating systems or security updates. Some updates can also affect application compatibility or equipment certification. Maintenance teams should therefore assess each change before installation.

Prioritize known vulnerabilities, exposed services, and assets with important operational roles. When direct patching is not possible, compensating controls such as segmentation, restricted access, and monitoring may reduce risk.

Maintain approved backups and recovery procedures. Test software and configuration changes outside production where feasible, and coordinate any required downtime with operations personnel.

Practical OT Security Checklist

• Build and maintain an inventory of legacy OT assets.
• Document network connections and required communication flows.
• Separate operational zones and restrict unnecessary traffic.
• Use controlled gateways for data exchange with external systems.
• Secure, authorize, and log remote access sessions.
• Assess vulnerabilities and document compensating controls.
• Maintain configuration backups and tested recovery procedures.

Modernization Does Not Have to Be Immediate

Legacy system security is often a long-term engineering program rather than a single replacement project. Some devices can be protected through network changes and access controls, while others may need staged migration because of technical limitations or operational risk.

Prioritize improvements according to asset criticality, exposure, business impact, and available recovery options. A practical roadmap can reduce risk without forcing a disruptive plant-wide replacement.