From Network Boundaries to Built-In Security: Expanding Zero Trust by Design

Zero Trust isn’t a new concept in operational technology (OT) security. For years, we’ve used it to justify network segmentation, isolate PLCs, and lock down traffic between critical devices. But OT environments are rapidly changing. The rise of smart sensors, cloud integrations, and remote access means that network boundaries alone aren’t enough. It’s time to expand how we think about Zero Trust and evolve it into a secure-by-design approach.

Originally, Zero Trust in OT focused on minimizing trust between systems on the same network. Firewalls, VLANs, and DMZs became the standard tools. The principle was simple: never trust, always verify. But while that worked well from a network segmentation process, today’s connected environments demand more. Verification is a critical piece that's often overlooked or not possible within OT environments.

The principle was simple: never trust, always verify. But while that worked well from a network segmentation process, today’s connected environments demand more.

Now, attackers target users, exploit endpoints, and move laterally through unmanaged assets. The traditional perimeter doesn’t stop them, especially when OT teams are juggling legacy systems that weren’t built for security in the first place. That’s why modern Zero Trust must go beyond segmentation.

What does that look like?

It means rethinking what we protect and how. We need to apply Zero Trust principles across identity, devices, applications, and data: not just the network. That’s the bridge to secure-by-design: embedding protection into every layer of the architecture from day one.

Here’s where to start:

• Map access beyond the network🗺️ Who’s logging in? From where? With what device? Identity is now just as important as IP.

• Enforce least privilege 🫸 Admin rights shouldn’t be a default. Only give access to what’s needed, and only for as long as it’s needed.

• Secure endpoints 🔐 Patching, Endpoint Detection and Response (EDR) tools, and hardening are essential, especially in OT, where those systems stay in place for years.

• Build in monitoring and response 💬 It’s not just about preventing attacks. It’s about detecting and containing them fast.

• Align teams ✅ IT, OT, and Security must be on the same page. Silos are the enemy of Zero Trust and secure-by-design strategies alike.

The end goal? Systems that assume compromise and are resilient by design. In other words, your architecture itself helps enforce security, even when people make mistakes or attackers get in.

Expanding Zero Trust into secure-by-design isn’t about replacing what you’ve done; it’s about building on it. Segmentation still matters. So does protocol filtering. However, we also need to secure the people and devices at the edges where decisions are made and data is generated.

Expanding Zero Trust into secure-by-design isn’t about replacing what you’ve done; it’s about building on it.

As we move beyond network segmentation toward holistic resilience, the real challenge is aligning people, processes, and technology from the ground up. We’d love to hear about your experiences, concerns, and strategies. What do you see as the biggest gaps in OT security today, and how are you working to close them?

Check out the full article on Zero Trust on our blog.

David Smit, OT Architect at Interstates