What is the difference between the Purdue Model and Zero Trust?

How the two models solve different problems

The simplest distinction is that the Purdue Model describes where systems sit and how traffic is segmented in an industrial environment, while zero trust describes how every access request should be evaluated. Purdue is an architecture and zoning model. Zero Trust is an access and policy model. That means they answer different questions: one organises the environment, the other governs decisions inside it.

In practice, Purdue is about dividing enterprise IT, operations technology, control systems, and field equipment into layers and conduits so the environment has a defensible structure. Zero Trust does not replace that structure, it changes the assumption behind it. A connection is not trusted just because it is inside a certain zone or came through a known path. Access still has to be explicitly authorised, continuously evaluated, and constrained by policy.

A useful way to think about it is that Purdue can shape the boundaries, while Zero Trust can shape the rules inside and across those boundaries. An industrial network can use segmentation and still be weak if anything on the “trusted” side can move laterally without strong verification. A Zero Trust approach can reduce that implicit trust, but it still needs a physical or logical architecture to enforce the policy cleanly.

Where Purdue still matters in a Zero Trust design

Industrial environments are not flat office networks. Control loops, safety systems, historians, engineering workstations, remote access paths, and vendor connections all create different risk profiles, so a layered model still helps with scoping and containment. The Purdue Model remains useful because it reflects operational dependencies and the reality that not every device or protocol can be treated the same way.

Zero Trust becomes most valuable when it is applied to the access decisions between those layers. That includes human users, remote administrators, service connections, and machine-to-machine traffic. A strong industrial design usually combines zoning with identity-aware access, device posture checks, least privilege, and per-session or per-request policy enforcement. OT security guidance on the Purdue Model is useful because it shows how segmentation, conduits, and industrial protocol controls fit the operational environment.

Zero Trust is also especially relevant when a network boundary is no longer the main control. Remote maintenance, cloud-connected monitoring, third-party support, and modern industrial gateways often bypass the old assumption that internal traffic is inherently safe. That is why Zero Trust Identity guidance is a good complement: it shows how policy decisions depend on identity, device state, and continuous verification rather than location alone.

For workload and service-to-service access, the same idea applies. Industrial and adjacent systems increasingly rely on certificates, service identities, and mutually authenticated channels, so SPIFFE and SPIRE help illustrate how workload identity can support Zero Trust without flattening the environment into a single trust zone.

Why people confuse segmentation with Zero Trust

Both models try to reduce blast radius, but they do it differently. Purdue reduces blast radius primarily by arranging systems into layers and limiting interconnections. Zero Trust reduces blast radius by denying implicit trust and requiring policy checks for access, even when traffic crosses an internal boundary. That means segmentation is one tool in a Zero Trust design, not the entire design.

The common mistake is to assume that a well-drawn network diagram equals strong security. A segmented network can still permit overbroad access, shared credentials, unmanaged remote paths, or overly permissive service accounts. Conversely, a Zero Trust policy without good network boundaries can become hard to enforce and difficult to audit in industrial settings. The practical answer is not either-or, it is using segmentation to organise the environment and Zero Trust to govern access through it.

That combination is why the most useful reference point for many readers is the NIST model for Zero Trust Architecture. NIST SP 800-207 Zero Trust Architecture formalises the idea that trust should be continuously evaluated rather than inherited from network position.

Risk and Threat Considerations

In industrial environments, the risk is not just unauthorized access, it is access that can affect process integrity, safety, or availability. If segmentation is treated as a substitute for verification, an attacker who reaches one trusted segment may move laterally into control paths, engineering tools, or remote maintenance channels. If Zero Trust is treated as a policy slogan without enforceable boundaries, access decisions may be inconsistent and impossible to operationalise across legacy systems.

Failure mechanism: Overreliance on zone membership, shared trust domains, or static network placement can allow a compromised account, device, or vendor path to inherit more reach than intended, especially where OT systems still depend on legacy protocols or long-lived access paths.

Impact: The result can be expanded blast radius, degraded containment, and a higher chance that a single compromise reaches control assets, engineering workstations, or safety-relevant systems.

Practitioner Guidance

What to verify: Check whether your architecture separates environment layout from access decisions. If the answer is no, you probably have segmentation, not Zero Trust. The strongest implementations make policy depend on identity, device posture, and explicit authorization while still preserving industrial zones and conduits.

Decision rule: Use Purdue to define where control boundaries and dependencies exist, then use Zero Trust to decide who or what may cross them, under what conditions, and for how long. If a connection must remain broad to keep the process running, treat that as a risk exception and monitor it tightly rather than calling it “trusted”.

Practitioner takeaway: Purdue tells you how the industrial environment is structured; Zero Trust tells you how to stop that structure from becoming an assumption of trust.

Related resources from NHI Mgmt Group

• What is the difference between zero trust for users and zero trust for NHIs?
• What is the difference between JIT access and Zero Trust for NHIs?
• What is the difference between zero trust and a traditional VPN model?
• What is the difference between managing human identities and non-human identities under a Zero Trust model?