What breaks when OT security relies on implicit trust between connected systems?

Why Implicit Trust Is Dangerous in OT

Implicit trust in OT creates a control gap between what is connected and what is actually trustworthy. Once a device, engineer workstation, remote access path, or vendor link is treated as inherently safe, the environment becomes easier to traverse after a single compromise. That is especially dangerous in plants and critical infrastructure where uptime pressures often delay segmentation changes, monitoring improvements, or authentication hardening. NIST’s Cybersecurity Framework 2.0 is useful here because it reinforces the need to identify, protect, detect, respond, and recover rather than assume trust inside the network.

NHIMG research shows how identity failure amplifies this problem: The Ultimate Guide to Non-Human Identities reports that 80% of identity breaches involved compromised non-human identities such as service accounts and API keys. In OT, those identities often bridge legacy controllers, historians, edge gateways, and remote support tools, so one weak link can expose more than the initial system. In practice, many security teams discover implicit trust only after a vendor path or engineering account has already been used to move laterally.

How It Works in Practice

OT networks often connect systems for telemetry, patching, remote maintenance, and production orchestration, but the trust relationship is frequently broader than the operational need. A workstation that can reach a controller may also reach adjacent engineering assets, shared file services, or protocol bridges. If the environment relies on flat routing, shared credentials, or permissive allowlists, attackers can reuse that trust to pivot from one zone to another.

Current best practice is to make trust explicit and measurable. That usually means:

• segmenting OT zones and conduits so access is purpose-built, not inherited
• treating every connection as authenticated and authorized at the point of use
• using short-lived credentials or JIT access for remote support and admin tasks
• binding machine-to-machine access to workload identity instead of static shared secrets
• logging and validating each tool call, session, and protocol path for abnormal reuse

For identity-heavy environments, NHIMG’s Schneider Electric credentials breach illustrates how exposed credentials can create broader operational risk when trust is too broad. SPIFFE-style workload identity and policy engines such as OPA are increasingly used to prove what a system is and what it may do, but guidance is still evolving for brownfield OT. This is why the best starting point is often to map every connected system, every credential, and every maintenance path against actual production necessity, then remove standing access wherever possible. These controls tend to break down when legacy controllers must communicate through shared gateways because the gateway becomes a high-trust exception that is difficult to constrain.

Where the Model Breaks Down in Real OT Environments

Tighter trust controls often increase operational overhead, requiring organisations to balance safety and availability against speed of maintenance and incident response. That tradeoff is real in OT, where patch windows are narrow, some devices cannot support modern authentication, and protocol dependencies may be undocumented. There is no universal standard for this yet, so current guidance suggests prioritising the highest-risk trust paths first rather than attempting a full redesign in one step.

Two edge cases matter most. First, vendor remote access often arrives as a temporary exception that becomes permanent in practice, especially when teams avoid revocation to reduce downtime. Second, protocol translation devices can mask which endpoint is actually trusted, which makes segmentation look stronger than it is. NHIMG’s Scania Supply Chain Data Breach is a useful reminder that connected third parties can extend the blast radius well beyond the first system touched. The practical goal is not zero connectivity, but zero implied trust: every connection should have an owner, a purpose, a scope, and a revocation path.

Related resources from NHI Mgmt Group

• What breaks when OT security relies only on detection tools?
• What breaks when legacy identity systems are ignored in OT zero trust programmes?
• How should security teams govern AI vendors connected to OT systems?
• What breaks when organisations try to enforce zero trust uniformly across OT and legacy industrial systems?