Who is accountable for authentication when a gateway relays agent traffic?

The upstream management server remains accountable because it makes the accept or reject decision. The gateway can forward headers and relay messages, but it should not become the policy authority unless the architecture explicitly assigns that responsibility.

Why This Matters for Security Teams

Authentication responsibility is a control ownership question, not just a routing question. When a gateway relays agent traffic, teams can mistakenly treat the relay layer as the trust boundary and assume header forwarding is enough. That is risky because acceptance decisions, identity binding, and audit accountability should remain with the component that actually evaluates policy. The OWASP Agentic AI Top 10 highlights how agentic systems fail when trust is distributed without clear authority, especially where tool access and delegated actions are involved.

For security teams, the practical issue is that a gateway often sees traffic first, but seeing traffic first is not the same as owning authentication. If a relay can alter, drop, or mint identity context, then authentication and non-repudiation become ambiguous. That ambiguity creates gaps in incident response, access review, and post-incident forensics. NHI and agent governance become relevant here because the upstream management server is effectively the identity authority for the agent, while the gateway is only a transport or mediation layer unless policy says otherwise. In practice, many security teams encounter this failure only after a relay has been trusted as an authority and an access decision is already in question.

How It Works in Practice

The cleanest model is to separate transport, policy, and identity assertion. The gateway should relay requests, preserve integrity where possible, and pass along identity context, but the upstream management server should perform the actual accept or reject decision. That means authentication evidence, session state, service identity, or agent attestation should be validated by the authoritative control plane, not inferred from the gateway’s presence alone. This aligns with the governance approach described in the NIST AI Risk Management Framework, which expects clear accountability for AI system behaviour and decision-making.

In operational terms, teams usually implement this with a few guardrails:

• The gateway forwards headers or tokens, but does not rewrite identity claims unless it is explicitly designed as an identity broker.
• The management server validates the agent’s identity, policy eligibility, and session context before granting access to tools or data.
• Logs record both the relay path and the authority that made the decision, so audit trails show who authenticated whom.
• Mutual TLS, signed tokens, or short-lived credentials help reduce the risk that forwarded identity context is replayed or altered.

This design also maps well to control expectations in NIST SP 800-53 Rev 5 Security and Privacy Controls, especially where identity assurance, access enforcement, logging, and separation of duties are required. Where agent systems are involved, the issue is not just user authentication but whether the relay can safely represent an autonomous software entity without becoming the trust authority itself. These controls tend to break down when legacy API gateways both terminate sessions and mint identity claims because the architecture blurs the distinction between relay, broker, and policy decision point.

Common Variations and Edge Cases

Tighter authentication separation often increases integration overhead, requiring organisations to balance simpler routing against stronger accountability. The tradeoff is especially visible when a gateway is expected to do edge protection, protocol translation, and identity mediation at the same time. Current guidance suggests that this can be acceptable only if the gateway is formally designated as an identity broker and its trust role is documented in policy, architecture, and logging. Otherwise, best practice is to keep authentication authority upstream and keep relay behaviour narrow.

There are a few edge cases where the answer becomes more nuanced. If the gateway performs token exchange, certificate termination, or identity enrichment, it may participate in authentication, but participation is not the same as final accountability. If multiple services share the same relay, each trust relationship should be explicit, because delegated access for one agent should not imply authentication authority for all. In high-assurance environments, teams should also review whether gateway logs include enough evidence for reconstruction, especially when the original management server is offline or asynchronous. For agentic deployments, the CSA MAESTRO agentic AI threat modeling framework is useful for distinguishing control-plane authority from message-plane mediation, while the MITRE ATLAS adversarial AI threat matrix helps teams model relay abuse, impersonation, and identity spoofing paths.

Related resources from NHI Mgmt Group

• Who is accountable when an exposed AI agent gateway leaks secrets and chat history?
• Who is accountable when intercepted credentials are reused across cloud and agent traffic?
• Who is accountable when an AI coding agent sends traffic to the wrong system?
• Who is accountable when DNS spoofing affects authentication traffic?