How should defence and critical infrastructure teams implement Zero Trust without treating it as a one-time architecture project?

Teams should treat Zero Trust as an enterprise operating model, not a single deployment. Start by aligning governance, funding, training, and technical controls across users, devices, applications, data, networks, and automation. The DoD approach shows that success depends on coordinated implementation, continuous verification, and iterative improvement across mission and business systems, rather than relying on network location or inherited trust.

Make Zero Trust a Program, Not a Project

zero trust works best when teams treat it as a long-lived operating model with governance, funding, ownership, and measurement, not as a one-off redesign of the network. That distinction matters in defence and critical infrastructure because the hardest part is not designing a policy set once, but keeping it aligned as missions, users, devices, applications, and suppliers change.

The most practical way to start is to define an enterprise scope that spans users, devices, applications, data, networks, and automation, then map which teams own each control plane. The NIST Zero Trust Architecture guidance is useful here because it frames Zero Trust around policy decision and enforcement, continuous verification, and explicit trust rules rather than inherited network location, and the DoD’s implementation approach reinforces that this has to be coordinated across mission systems and business systems. For workload and machine access, the same idea extends naturally into NHIMG’s Ultimate Guide to NHIs, because service accounts, API keys, certificates, and other non-human access paths are part of the same trust model.

Implementation sequence matters. Establish the target operating model first, then phase controls where exposure is highest, such as remote access, privileged access, and east-west segmentation, before expanding to broader application and data flows. A programmatic approach also makes it easier to keep policy, identity, endpoint posture, and logging aligned as systems evolve rather than forcing teams to rework the architecture every time a new mission capability appears.

What Changes in Critical Environments

Defence and critical infrastructure environments usually have legacy systems, operational technology, segmented vendor access, and mission continuity requirements that make “big bang” Zero Trust rollouts unrealistic. The correct question is not whether every system can be transformed immediately, but which trust assumptions can be removed first without breaking availability, safety, or operational command and control.

That is why funding and governance need to be tied to lifecycle management, not just initial deployment. Teams should expect recurring work in policy tuning, authentication hardening, device assurance, credential hygiene, exception review, and telemetry validation. If those activities are not explicitly owned, Zero Trust becomes a set of disconnected tools that look modern but still leave broad implicit trust in place. The DoD model is helpful precisely because it treats Zero Trust as a coordinated enterprise transition rather than a point product purchase. For the infrastructure identity side of that transition, The 2026 Infrastructure Identity Survey shows how quickly access governance can drift when teams rely on static credentials and over-privileged systems.

In practice, the main design trade-off is between strictness and operational continuity. Teams often need compensating controls, staged enforcement, and exception handling for legacy assets that cannot yet support modern policy checks. The important discipline is to make those exceptions visible, time-bound, and measured so they do not become permanent trust backdoors.

Risk and Threat Considerations

Zero Trust programs fail when organisations confuse architectural intent with actual enforcement. The usual failure modes are overreliance on network segmentation alone, incomplete identity coverage, weak device posture checks, and unmanaged exceptions for legacy or third-party access. In critical environments, those gaps create a path for lateral movement, privilege abuse, and persistence even when the original deployment looks compliant.

Failure mechanism: Attackers and insiders exploit any remaining implicit trust, such as static credentials, broad access paths, stale device trust, or vendor connections that were never brought under continuous verification. If policy decisions are not enforced consistently across systems, the environment retains islands of inherited trust that can be abused after initial compromise.

Impact: The result is broader blast radius, harder containment, and more difficult recovery, especially where mission systems or operational technology depend on availability and cannot tolerate disruptive retrofits. Over time, the gap between policy and practice also undermines executive confidence in the program and weakens future investment.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

OWASP Non-Human Identity Top 10 address the attack and risk surface, while NIST CSF 2.0, NIST SP 800-63, NIST Zero Trust (SP 800-207) and CIS Controls v8 set the governance and control requirements practitioners need to meet.

Framework	Control / Reference	Relevance
NIST CSF 2.0	GV — Govern	Zero Trust here requires enterprise governance, ownership, and funding across systems.
	PR.AC — Identity Management, Authentication, and Access Control	Zero Trust depends on continuous verification and least-privilege access enforcement.
	DE.CM — Security Continuous Monitoring	Iterative Zero Trust needs telemetry to verify policy enforcement and exception drift.
Recommendation — Assign governance ownership and funding for continuous Zero Trust operation. Enforce least-privilege access with continuous authentication and authorization checks. Continuously monitor enforcement points and exception paths for policy drift.
NIST SP 800-63	IAL/AAL/FAL — Identity Assurance, Authenticator Assurance, Federation Assurance	Zero Trust implementation hinges on strong identity assurance for users and systems.
Recommendation — Use strong identity and authenticator assurance for every access decision.
NIST Zero Trust (SP 800-207)	Policy Decision Point / Policy Enforcement Point — Policy Decision and Enforcement Architecture	Zero Trust operates through continuous policy decisions and enforcement, not static trust.
Recommendation — Implement explicit policy decision and enforcement points for every protected resource.
CIS Controls v8	6 — Access Control Management	Least privilege and controlled access paths are central to staged Zero Trust rollout.
	8 — Audit Log Management	Continuous verification requires logs that prove policy decisions and access behaviour.
Recommendation — Tighten access control paths and remove unnecessary trust relationships. Centralize and retain audit logs for Zero Trust enforcement verification.
OWASP Non-Human Identity Top 10	NHI-01 — Secret Sprawl and Credential Exposure	Zero Trust in enterprise environments must cover machine and automation credentials as well as users.
	NHI-03 — Privilege and Access Scope	Over-privileged non-human access is a common way Zero Trust programs fail in practice.
	NHI-09 — Lifecycle and Offboarding	Zero Trust needs ongoing revocation and rotation, not one-time deployment decisions.
Recommendation — Inventory and reduce exposed machine credentials that bypass Zero Trust controls. Scope non-human access to the minimum privileges required for each workflow. Automate rotation, revocation, and offboarding for machine identities and secrets.

Practitioner Guidance

What to prioritise: Start with the access paths that create the greatest blast radius, typically privileged users, remote vendor access, and automation credentials that touch production systems. Those are the places where continuous verification and least privilege deliver the fastest risk reduction.

What to verify: Confirm that every control domain has a named owner, a measurable target state, and an exception process. If you cannot show who is accountable for policy, enforcement, and review, the program is still a design exercise, not an operating model.

Common mistake: Treating microsegmentation or identity tooling as the whole program. Zero Trust only becomes durable when governance, telemetry, and lifecycle management keep pace with technical rollout, otherwise teams simply move trust from one layer to another.

Practitioner takeaway: The right success metric is not whether Zero Trust has been “deployed”, it is whether trust is continuously re-evaluated, exceptions are bounded, and the program can adapt as missions, systems, and access paths change.