A realistic OT security guide for industrial environments

Industrial cybersecurity has become a key concern for IT and security teams across manufacturing and critical infrastructure. At the same time, many industrial environments still depend on legacy OT systems that were never designed with modern cybersecurity requirements in mind. These systems often run reliably for decades and replacing them is usually not an option due to cost, certification requirements, or the risk of production downtime.

The challenge, therefore, is not how to build a greenfield OT environment, but how to secure existing systems in a realistic and sustainable way. This guide focuses on practical measures that help organizations improve OT security and reduce cyber risks without replacing legacy OT systems.

Why legacy OT systems remain a critical risk factor

Legacy OT systems were developed for isolated environments with little or no connectivity to external networks. Security mechanisms such as authentication, encryption, or granular access control were often not part of the original design. Today, however, these systems are increasingly connected to IT networks, remote access solutions, and external service providers.

This shift significantly expands the attack surface. At the same time, legacy OT systems are difficult to update or harden without risking availability. From an industrial cybersecurity perspective, this means that traditional IT security approaches are often not applicable. OT security must work around technical and operational limitations rather than trying to eliminate them .

OT environments follow different engineering and lifecycle principles than IT: assets often run for decades (15 30+ years) and change windows are limited because downtime is costly and modifications may require safety re-validation or recertification. Many devices also have limited resources, use proprietary protocols, and depend on vendor support, which makes patching difficult. That is why OT security must prioritise availability and safety and use compensating controls around legacy systems rather than frequent updates.

Building a realistic security baseline through visibility

Effective OT security starts with understanding the existing environment. Without visibility, security measures remain reactive and incomplete. Many organizations underestimate how many legacy OT systems are connected to their networks or how data actually flows between IT and OT.

A structured assessment helps establish a baseline and answer fundamental questions, such as:

• Which legacy OT systems are connected to the network and for what purpose?
• Which assets are present in the OT network, and which of them require protection?
• Which communication paths exist between IT, OT, and external networks?
• Which systems are critical for safety, compliance, or production continuity?

Using frameworks such as IEC 62443 helps structure this analysis and supports a risk-based prioritization instead of a one-size-fits-all approach.

Network segmentation as a cornerstone of industrial cybersecurity

For legacy OT systems, network segmentation is one of the most effective and realistic security measures available. Instead of trying to secure individual systems that cannot be changed, segmentation focuses on controlling communication and limiting exposure.

By dividing industrial networks into clearly defined zones and controlling traffic between them, organizations can significantly reduce the impact of cyber incidents. Segmentation helps ensure that a compromise in one area does not automatically affect the entire production environment. This makes it a foundational element of industrial network security and a key recommendation in OT security standards.

In practice, segmentation is typically implemented with network infrastructure components rather than changes on the legacy devices themselves. Common building blocks include managed industrial switches (for VLANs and port-based controls), industrial routers or security gateways to separate cells and lines, and firewalls to enforce allow-listed traffic between zones and conduits.

Improving access control without modifying legacy systems

Many legacy OT systems cannot enforce modern authentication or authorization mechanisms. However, this does not mean access control cannot be improved. In practice, access is often governed by the surrounding infrastructure rather than the system itself.

Typical measures include clearly defined access paths into the OT environment, controlled remote access, and centralized user management outside the legacy system. Instead of unrestricted connectivity, access is limited to what is operationally required. This significantly reduces the risk of unauthorized access while maintaining maintainability and availability.

Defense in depth as a practical OT security strategy

Because no single control can adequately protect legacy OT systems, defense in depth is a central principle of OT cybersecurity. The idea is not to rely on one protective measure, but to combine multiple layers that complement each other.

In practice, this typically includes:

• Network segmentation to limit communication paths
• Access control at network and system boundaries
• Monitoring and logging to detect anomalies
• Secure configuration and hardening of surrounding components

Even if a legacy OT system remains vulnerable, these layers reduce the likelihood of successful attacks and limit their potential impact.

Using IEC 62443 as a pragmatic framework

IEC 62443 provides a useful structure for addressing industrial cybersecurity challenges in existing environments. Importantly, the standard recognizes that legacy systems are part of reality and promotes a gradual, risk-based approach.

By defining security zones, assigning appropriate security levels, and implementing technical and organizational measures step by step, organizations can continuously improve their security posture. This approach also helps align IT and OT teams by establishing a shared language and common security objectives.

Collaboration between IT and OT as a success factor

Securing legacy OT systems is not only a technical challenge, but also an organizational one. Sustainable OT security requires close collaboration between IT, OT, and security teams. Each group brings different priorities and constraints, and effective solutions must balance security with operational reliability.

When security measures are designed to support stable and predictable operations, industrial cybersecurity becomes an enabler rather than a disruption.

Did you know?

At Phoenix Contact, we offer an industrial cybersecurity portfolio that builds on the same ideas discussed above. By applying IEC 62443 principles such as network segmentation, controlled access, and defense in depth, we help industrial companies strengthen the security of legacy OT systems without the need for replacement.

Learn more here: https://phoe.co/industrial-security