Because a registry only tells an agent what exists and where it is. Without enforcement in the traffic path, discovery can become unaudited access, which leaves organisations unable to limit misuse, revoke access consistently, or prove which agent used which tool.
Why a registry without enforcement becomes a security blind spot
An MCP registry is useful for discovery, but discovery alone does not create control. If clients can learn about tools and connect to them without a policy check in the request path, the registry becomes an index of available access rather than a guardrail. That is where misuse, shadow access, and weak attribution start.
A registry is also structurally weaker than enforcement because it lives in the control plane, while actual use happens in the data path. If those layers are separated, an agent can still call a tool through another route, retain old access after a registry update, or exploit a stale registration that nobody is actively validating.
In practice, the security question is not whether the registry is accurate, but whether every tool call is evaluated at runtime against current identity, scope, and policy. Without that step, the organisation can list tools, but it cannot reliably decide who may use them, under what conditions, or whether a particular invocation should be blocked.
What runtime enforcement changes in the MCP trust model
runtime enforcement turns registry data into an executable control. The registry may tell an agent what exists, but enforcement decides whether the current session, token, client, tool, and context are allowed to proceed. That is what prevents the registry from becoming a convenience layer that silently expands access.
This matters because MCP environments can include delegated access, token forwarding, and tool mediation. The MCP authorization specification is explicit that servers should behave as resource servers with audience-bound tokens, which is the kind of runtime control that stops a registry from acting like implicit authorisation.
At the architecture level, enforcement also reduces confusion between discovery and permission. A registry can support onboarding, cataloguing, and routing, but it should not be the sole source of truth for whether an agent may execute a tool. If it is, access decisions drift from policy enforcement into metadata management, and metadata is easier to stale, copy, or bypass.
Why the failure mode matters for agents, tools, and revocation
The most serious failure mode is that a registry gives the appearance of governance while actual access remains unmediated. That creates a false sense of control, because teams may believe that removing an entry or changing a listing is enough to revoke access, when in reality the tool can still be reached through an existing token, cached configuration, or alternate path.
That same gap makes attribution and audit weaker. If the runtime does not verify and log each tool invocation, the organisation may know that a tool was advertised, but not which agent used it, whether the invocation was permitted, or what context was presented at the time. Once multiple agents or clients are involved, that missing evidence becomes a governance problem as much as a security one.
The risk grows when registries are treated as trust anchors for agent selection. In that model, the agent learns what exists and then assumes the existence of a listed tool implies legitimacy. The safer pattern is to treat the registry as a catalog and require the enforcement layer to make every actual request conditional on policy, scope, and provenance.
Risk and Threat Considerations
When a registry is not paired with runtime enforcement, it can expose more than discovery data. It can become a shortcut into sensitive tools, especially when agents or integrators assume that catalogued availability implies permitted use, or when stale listings remain reachable after access should have been removed.
Failure mechanism: The gap between advertised capability and enforced authorisation lets a client discover, cache, or replay access paths that were never validated at the moment of use, or that should have been revoked after the registry changed.
Impact: Organisations lose the ability to reliably limit misuse, revoke access consistently, and prove which agent used which tool, which weakens containment, incident response, and accountability.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP Agentic AI Top 10 and OWASP Non-Human Identity Top 10 address the attack and risk surface, while NIST SP 800-53 Rev 5 sets the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP Agentic AI Top 10 | ASI03 — Identity & Privilege Abuse | Registry-only access can be abused when agent identity and privilege are not enforced at runtime. |
| ASI02 — Tool Misuse | A registry without enforcement can let agents invoke tools in unintended ways. | |
| ASI07 — Insecure Inter-Agent Communication | Registry-driven discovery can mislead agent-to-tool trust relationships without enforced verification. | |
| Recommendation — Enforce runtime authorization checks before any agent tool call. Gate tool execution with policy checks, not catalog visibility. Validate every inter-agent or agent-to-tool request before allowing execution. | ||
| OWASP Non-Human Identity Top 10 | NHI-04 — Insecure Authentication | Runtime enforcement is needed so registry discovery does not bypass authentication at use time. |
| NHI-05 — Overprivileged NHI | Without enforcement, registry-listed tools may remain accessible with excessive privileges. | |
| Recommendation — Require live authentication checks for each tool invocation. Constrain tool access to the minimum scope needed at runtime. | ||
| NIST SP 800-53 Rev 5 | AC-3 — Access Enforcement | The question centers on enforcing access decisions at the point of use. |
| AU-2 — Event Logging | The answer depends on proving which agent used which tool. | |
| IA-5 — Authenticator Management | Registry-only discovery can leave stale credentials usable unless managed at runtime. | |
| Recommendation — Enforce access decisions at the runtime request path. Log each authorized tool invocation with actor and decision context. Rotate and revoke credentials independently of registry listings. | ||
Practitioner Guidance
What to verify: Confirm that tool access is denied unless the runtime can evaluate the current identity, token, scope, and policy at the moment of invocation. A registry entry should never be accepted as sufficient evidence of permission.
Decision rule: If a registry change does not immediately and measurably alter live access decisions, treat the registry as informational only and keep enforcement in the request path.
What good looks like: Removing or narrowing a tool in policy should block new calls right away, and every permitted call should produce an auditable record that ties the agent, the tool, and the enforcement decision together.
Practitioner takeaway: Discovery helps agents find tools, but only runtime enforcement stops discovery from becoming unaudited access.
Related resources from NHI Mgmt Group
- Why does broad agent access create confused-deputy risk in MCP workflows?
- Why do valid OAuth tokens still create risk for AI coding agents and MCP tools?
- Should telecoms prioritise runtime enforcement or broader IAM cleanup first for MCP risk?
- Why do custom MCP runtime layers create so much security and operational risk in production?
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Reviewed and updated by the NHIMG editorial team on October 11, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org