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A secure AI API gateway controls who may request work, where data may go, how much work is admitted and what may happen with the result. Proxying a JSON body to a model provider is only the transport layer. The security boundary must remain valid when requests are malformed, suppliers fail or a model proposes an unsafe action.
This guide develops a threat model and implementation review framework. It uses the repository's tested FastAPI teaching example as a small reference, while identifying the controls that example does not implement. It is not a claim that any service has passed a security audit.
Define trust boundaries first
Identify callers, identities, tenants, gateway workers, provider accounts, tool services and administrative interfaces. Mark where authentication occurs and which component authorizes each operation. A successful login does not authorize every model, document or tool.
Write concrete abuse cases: an anonymous client burns credits, a customer reads another tenant's cache, a caller chooses an internal URL, a model inserts an unauthorized tool action, or an upstream error leaks a credential. Controls should map to these cases rather than form a generic checklist detached from the architecture.
| Boundary | Required decision | Example evidence |
|---|---|---|
| Caller to gateway | Identity, tenant and operation permission | Negative authorization tests |
| Gateway to provider | Approved destination, credential and data policy | Fixed routing configuration and outbound tests |
| Gateway to tool | Authorized action and argument validation | Ownership checks and approval record |
| Response to client | Safe structure and permitted information | Schema/error fixtures |
| Operations and storage | Access, retention and redaction | Configuration review and recovery tests |
Authenticate and authorize independently
Use an identity mechanism appropriate to the product. A server-to-server token, user session and scoped end-user key have different lifecycles. Restrict privileges, rotate credentials and avoid logging bearer tokens. An operator's unrestricted provider secret belongs in a protected server environment.
Authorize tenant membership, model access, document scope and tool permissions on the server. Do not trust a client-provided tenant ID without binding it to the authenticated subject. Apply object-level authorization when retrieving data, not only at the top-level route.
The FastAPI tutorial uses a single shared teaching token and constant-time comparison. It tests rejection before provider invocation. That demonstrates one ordering property, not a complete session, revocation or multi-tenant identity system.
Keep destinations under server control
Accepting an arbitrary provider base URL can expose internal services or send credentials to an attacker-controlled host. Prefer a server-owned route identifier mapped to approved origins, models and processing policies. Review redirects, DNS/network controls and proxy behavior as part of that boundary.
The tested example fixes the HTTPS provider endpoint and disables redirects. Its request schema forbids extra fields, so callers cannot override the model or destination. A production gateway may support several providers, but each route should be an approved configuration rather than free-form caller input.
For AIx, verify the selected operation in current documentation. Compatibility does not establish every feature or privacy condition; use contract checks for those requirements.
Bound work before it becomes expensive
Enforce raw body-size limits at the edge, schema and field limits in the application, active-work limits in each service and account-level financial policy in durable storage. A text field maximum does not prevent a huge HTTP body from reaching a JSON parser.
Use a total request deadline and bounded queues. Restrict output, tool iterations and retries. An authenticated user can still abuse a service or trigger accidental loops. Rate limiting and Rust admission control illustrate distinct local controls, neither of which replaces a durable spending ledger.
Decide how reservations and usage reconciliation work when a provider times out. A local timeout does not prove zero upstream consumption. Avoid refunding or charging solely from an ambiguous client disconnect without a documented policy.
Treat model output as untrusted input
Validate response structure and length before downstream use. Escape generated content according to its rendering context. Never execute model-proposed code or URLs merely because they appear in a valid JSON response.
For tools, validate the action name, typed arguments, resource ownership and allowed transition. Require appropriate user approval for consequential actions. Bind approvals to the actual action and current state so an old approval cannot authorize a different later operation.
Prompt instructions can help communicate policy, but authorization must be enforced outside the model. A malicious document that tells the model to reveal secrets must not gain privileges through a tool call. Use the minimum data and credentials needed for each approved operation.
Make errors useful without exposing internals
Return stable client errors for authentication, validation, rejection, deadline and provider failure. Do not forward raw upstream exception bodies, headers or stack traces. Those can contain request content, account metadata or secrets.
Keep restricted diagnostics with request and attempt identifiers, sanitized error class and relevant timing. Default to avoiding prompts and generated answers in ordinary logs. If a specific investigation requires content capture, use a reviewed access, retention and consent process.
OWASP's REST security guidance is a useful reference for interface controls. Applying a checklist is not proof of complete protection; verify the actual boundary with negative tests.
Test the provider boundary separately
Use deterministic fixtures for malformed responses, network failures and delayed work. The Python example's tests verify sanitized 502 and 504 responses, authorization, schema restrictions and local rate rejection. They do not certify the live provider's behavior.
Add a separate controlled live contract check before release: approved model, sanitized input, bounded spend and recorded configuration. Avoid using customer data to discover whether a new route works. A successful fixture and a successful live call are different evidence types.
Test cache isolation and revocation alongside routing. A gateway can authorize the live path correctly and still leak a result through an improperly shared cache. Caching and observability develops that boundary.
Prepare operations and change control
Restrict administrative configuration, audit changes to route mappings and secrets, and provide a way to disable a provider or tool quickly. Revalidate data-processing terms and task quality when adding a fallback route. An outage should not silently move restricted data to an unapproved destination.
Practice credential rotation and deployment rollback. Document what happens to in-flight work and partially completed external actions. Keep the incident process independent of one person's access or memory.
Build the smallest gateway whose trust boundaries you can test. Start with fixed destinations, narrow schemas, real authorization and bounded work; then add features only with corresponding failure and abuse tests.