Security

Hardening guidance for FastRMCP deployments. Pair with Middleware Cookbook and Transports Guide.

Auth Patterns

Bearer token: AuthMiddleware::bearer_token("secret") for quick protection; extend with custom middleware to validate per-request metadata.
Custom middleware: Inspect request.params.metadata (or _meta) for headers/tokens; return MCPError::InvalidRequest("unauthorized") on failure.
Client identity: Store authenticated user in Context metadata (context.set_metadata("user", json!(user_id)).await) for downstream tools.
Transport auth: For web transports, add Axum middleware before the MCP router to verify headers/cookies.

Use RateLimitMiddleware::new(limit, window) (simplified) or custom middleware with shared counters keyed by connectionId/client ID.
Apply stricter limits on expensive tools/resources; consider per-URI token buckets.

Restrict origins on web transports; replace permissive CorsLayer::new().allow_origin(Any) with explicit origins.
Expose only required transport(s) publicly; keep STDIO/internal transports behind firewalls or run on localhost.
Prefer TLS for SSE/WS in production; see docs/HTTP2.md for rustls setup.

Validate input types explicitly; use InvalidParams for user errors and avoid leaking internals.
Avoid returning sensitive metadata in tool results; strip in after_response middleware if needed.
For resources backed by filesystem/network, sandbox paths and timeouts; return ResourceError with generic messages on failure.

SSE: reject POSTs without connectionId when multiple connections exist; never echo other clients’ IDs.
WebSocket: server-initiated requests default to current connection; set metadata.connectionId to target explicitly and avoid cross-tenant leaks.

Rotate secrets without restart by storing them in shared State and reading inside middleware.
Log auth failures with minimal detail; avoid logging credentials.
Add health checks outside MCP if exposing HTTP servers; keep MCP methods private to trusted clients.

Related: Observability for logging failed auth/rate limits • Troubleshooting for common connection errors.