Implementing MCP Authorization in FastAPI

FastAPI can simplify implementation of the MCP authorization resource-server model through dependencies for JWT validation and scope enforcement, but developers must explicitly configure critical security controls. The guidance emphasizes upgrading the MCP Python SDK to version 1.23.0 or later and enabling transport security to mitigate CVE-2025-66416 DNS rebinding risks for unauthenticated localhost HTTP servers, while favoring Streamable HTTP over deprecated HTTP+SSE. It recommends publishing RFC 9728 protected-resource metadata at both well-known paths, naming an authorization server, and returning metadata pointers in authentication challenges. JWT validation should strictly verify signatures, issuer, expiration, required claims, allowed algorithms, and especially the configured canonical resource URI as the audience, since PyJWT does not validate audiences unless given an explicit audience parameter. Authentication failures, including invalid audiences, should return 401 responses, whereas valid tokens lacking required scopes should return 403 responses with an insufficient_scope challenge that identifies all needed scopes. The canonical resource URI must be configured rather than derived from request URLs behind proxies, where internal addresses can cause otherwise valid tokens to fail, and tenant identity should come from trusted token claims rather than request bodies. The guidance also advises testing wrong-audience and inadequate-scope cases in CI, avoiding token pass-through to upstream APIs, and recognizing that secure authorization depends as much on configuration, proxy handling, SDK versioning, and verification tests as on the FastAPI route code itself.

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