Why do unmanaged device dependencies make zero trust harder to enforce?

Because segmentation decisions depend on knowing which paths are necessary and which are merely convenient. If teams cannot distinguish required service flows from incidental connectivity, they either over-permit and leave attack paths open or over-restrict and break legitimate work. The risk rises in IoT, OT, and IoMT environments where dependencies are often hidden.

Why hidden dependencies undermine zero trust segmentation

zero trust segmentation only works when policy decisions reflect real application and device relationships, not assumptions. Hidden dependencies make that hard because the network team may not know which east-west paths are essential, which are incidental, and which exist only because of legacy shortcuts. That uncertainty turns segmentation into guesswork instead of controlled enforcement.

The result is a policy either too loose to reduce blast radius or too tight to support the workload. In practice, unmanaged dependencies often surface first in environments with IoT, OT, and IoMT traffic, where device behaviour is less transparent and service relationships are harder to inventory.

When that dependency map is incomplete, zero trust has no reliable baseline for least-privilege connectivity. The control can still exist, but it is no longer precise enough to separate required communication from convenience traffic.

How unmanaged dependencies create enforcement gaps

Unmanaged dependencies usually fail in two ways. First, teams over-permit to avoid outages, which leaves unnecessary paths open and weakens segmentation. Second, teams over-restrict without understanding service chains, which breaks legitimate operations and pushes users or operators to work around the policy.

Both outcomes erode trust in the program. Once stakeholders see failed connections or recurring exceptions, they begin to treat segmentation as a nuisance rather than a durable security control.

This is why dependency discovery is not a one-time inventory exercise. A path that looks nonessential today may become required after a firmware update, vendor change, failover event, or integration change. Zero trust policy has to keep pace with those changes or it slowly becomes inaccurate.

For a practical zero trust baseline, NIST SP 800-207 Zero Trust Architecture is the clearest external reference for tying access decisions to verified context rather than implicit network location. The same principle shows up in the Zero Trust Identity Guide, which is especially useful when segmentation policy must follow identity, workload, and device context together.

Why this is hardest in IoT, OT, and IoMT estates

IoT, OT, and IoMT environments create extra friction because discovery is often incomplete, tooling can be limited, and availability requirements are unforgiving. Many of these devices cannot be treated like standard endpoints, so operators hesitate to place them into strict policy regimes without strong evidence about their dependencies.

That makes unmanaged dependencies especially dangerous. If the organisation cannot prove the traffic pattern, it cannot confidently enforce microsegmentation, posture-based access, or per-service allow lists. The environment then drifts toward permissive exceptions, shared zones, or broad trust boundaries that weaken the zero trust model.

device trust matters here as much as identity trust. The Device Trust and Posture Guide is helpful where the access decision depends on whether the device is managed, healthy, and measurable. For device-heavy environments, the Device and IoT Identity Guide shows why strong device identity and attestation reduce ambiguity before segmentation policy is enforced.

Risk and Threat Considerations

Unmanaged dependencies increase both exposure and operational fragility. They create hidden pathways that attackers can exploit for lateral movement, while also making defenders more likely to accept broad exceptions that weaken containment.

Failure mechanism: If service flows are not mapped accurately, security teams cannot distinguish mandatory connectivity from accidental or legacy connectivity, so segmentation rules either overreach or underprotect.

Impact: Over-permissive rules preserve attack paths, and over-restrictive rules disrupt operations, encourage exception handling, and gradually reduce confidence in the zero trust program.

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 addresses the attack and risk surface, while NIST SP 800-53 Rev 5, 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 SP 800-53 Rev 5	AC-4 — Information Flow Enforcement	Directly governs restricting east-west flows to only necessary paths.
Recommendation — Enforce only approved information flows between segments and workloads.
NIST Zero Trust (SP 800-207)	Zero Trust Architecture	Core model for verifying each connection and eliminating implicit trust in network location.
Recommendation — Apply continuous verification and least-privilege policy to every access request.
CIS Controls v8	CIS-12 — Network Infrastructure Management	Supports controlling and segmenting network paths across mixed device environments.
Recommendation — Inventory, manage, and harden network paths and infrastructure devices.
OWASP Non-Human Identity Top 10	NHI-05 — Overprivileged NHI	Relevant where unmanaged service and device dependencies create excessive access paths.
Recommendation — Reduce standing access and trim permissions to the minimum required.

Practitioner Guidance

What to verify: Before tightening segmentation, confirm the dependency map with packet evidence, application owners, and change records. If a flow cannot be explained in business terms, treat it as suspicious until proven necessary.

Decision rule: If a connection is required for the workload to function, scope it narrowly and bind it to the smallest workable segment. If it exists only as convenience or historical residue, remove it rather than preserving it inside an exception.

Practitioner takeaway: Zero trust segmentation fails when policy is built on assumptions about connectivity instead of verified dependencies, so the first control objective is not tighter blocking, it is better visibility into what truly needs to talk to what.