--- title: OAuth Proxy sidebarTitle: OAuth Proxy description: Bridge traditional OAuth providers to work seamlessly with MCP's authentication flow. icon: share tag: NEW --- import { VersionBadge } from "/snippets/version-badge.mdx"; The OAuth proxy enables FastMCP servers to authenticate with OAuth providers that **don't support Dynamic Client Registration (DCR)**. This includes virtually all traditional OAuth providers: GitHub, Google, Azure, AWS, Discord, Facebook, and most enterprise identity systems. For providers that do support DCR (like Descope and WorkOS AuthKit), use [`RemoteAuthProvider`](/servers/auth/remote-oauth) instead. MCP clients expect to register automatically and obtain credentials on the fly, but traditional providers require manual app registration through their developer consoles. The OAuth proxy bridges this gap by presenting a DCR-compliant interface to MCP clients while using your pre-registered credentials with the upstream provider. When a client attempts to register, the proxy returns your fixed credentials. When a client initiates authorization, the proxy handles the complexity of callback forwarding—storing the client's dynamic callback URL, using its own fixed callback with the provider, then forwarding back to the client after token exchange. This approach enables any MCP client (whether using random localhost ports or fixed URLs like Claude.ai) to authenticate with any traditional OAuth provider, all while maintaining full OAuth 2.1 and PKCE security. For providers that support OIDC discovery (Auth0, Google with OIDC configuration, Azure AD), consider using [`OIDC Proxy`](/servers/auth/oidc-proxy) for automatic configuration. OIDC Proxy extends the OAuth proxy to automatically discover endpoints from the provider's `/.well-known/openid-configuration` URL, simplifying setup. ## Implementation ### Provider Setup Requirements Before using the OAuth proxy, you need to register your application with your OAuth provider: 1. **Register your application** in the provider's developer console (GitHub Settings, Google Cloud Console, Azure Portal, etc.) 2. **Configure the redirect URI** as your FastMCP server URL plus your chosen callback path: - Default: `https://your-server.com/auth/callback` - Custom: `https://your-server.com/your/custom/path` (if you set `redirect_path`) - Development: `http://localhost:8000/auth/callback` 3. **Obtain your credentials**: Client ID and Client Secret 4. **Note the OAuth endpoints**: Authorization URL and Token URL (usually found in the provider's OAuth documentation) The redirect URI you configure with your provider must exactly match your FastMCP server's URL plus the callback path. If you customize `redirect_path` in the OAuth proxy, update your provider's redirect URI accordingly. ### Basic Setup Here's how to implement the OAuth proxy with any provider: ```python from fastmcp import FastMCP from fastmcp.server.auth import OAuthProxy from fastmcp.server.auth.providers.jwt import JWTVerifier # Configure token verification for your provider # See the Token Verification guide for provider-specific setups token_verifier = JWTVerifier( jwks_uri="https://your-provider.com/.well-known/jwks.json", issuer="https://your-provider.com", audience="your-app-id" ) # Create the OAuth proxy auth = OAuthProxy( # Provider's OAuth endpoints (from their documentation) upstream_authorization_endpoint="https://provider.com/oauth/authorize", upstream_token_endpoint="https://provider.com/oauth/token", # Your registered app credentials upstream_client_id="your-client-id", upstream_client_secret="your-client-secret", # Token validation (see Token Verification guide) token_verifier=token_verifier, # Your FastMCP server's public URL base_url="https://your-server.com", # Optional: customize the callback path (default is "/auth/callback") # redirect_path="/custom/callback", ) mcp = FastMCP(name="My Server", auth=auth) ``` ### Configuration Parameters URL of your OAuth provider's authorization endpoint (e.g., `https://github.com/login/oauth/authorize`) URL of your OAuth provider's token endpoint (e.g., `https://github.com/login/oauth/access_token`) Client ID from your registered OAuth application Client secret from your registered OAuth application A [`TokenVerifier`](/servers/auth/token-verification) instance to validate the provider's tokens Public URL where OAuth endpoints will be accessible, **including any mount path** (e.g., `https://your-server.com/api`). This URL is used to construct OAuth callback URLs and operational endpoints. When mounting under a path prefix, include that prefix in `base_url`. Use `issuer_url` separately to specify where auth server metadata is located (typically at root level). Path for OAuth callbacks. Must match the redirect URI configured in your OAuth application Optional URL of provider's token revocation endpoint Issuer URL for OAuth authorization server metadata (defaults to `base_url`). When mounting your MCP server under a path prefix (e.g., `/api`), set this to your root-level URL to avoid 404 logs during OAuth discovery. MCP clients try path-scoped discovery first per RFC 8414, which will fail if your auth server metadata is at the root level. **Example with mounting:** ```python auth = GitHubProvider( base_url="http://localhost:8000/api", # OAuth endpoints under /api issuer_url="http://localhost:8000" # Auth server metadata at root ) ``` Without `issuer_url`, clients will attempt `/.well-known/oauth-authorization-server/api` (404) before falling back to `/.well-known/oauth-authorization-server` (success). Setting `issuer_url` to the root eliminates the 404 attempt. **When to use:** - **Default (`None`)**: Use `base_url` as issuer - simple deployments at root path - **Root-level URL**: Mounting under a path prefix - avoids 404 logs See the [HTTP Deployment guide](/deployment/http#mounting-authenticated-servers) for complete mounting examples. Optional URL to your service documentation Whether to forward PKCE (Proof Key for Code Exchange) to the upstream OAuth provider. When enabled and the client uses PKCE, the proxy generates its own PKCE parameters to send upstream while separately validating the client's PKCE. This ensures end-to-end PKCE security at both layers (client-to-proxy and proxy-to-upstream). - `True` (default): Forward PKCE for providers that support it (Google, Azure, AWS, GitHub, etc.) - `False`: Disable only if upstream provider doesn't support PKCE Token endpoint authentication method for the upstream OAuth server. Controls how the proxy authenticates when exchanging authorization codes and refresh tokens with the upstream provider. - `"client_secret_basic"`: Send credentials in Authorization header (most common) - `"client_secret_post"`: Send credentials in request body (required by some providers) - `"none"`: No authentication (for public clients) - `None` (default): Uses authlib's default (typically `"client_secret_basic"`) Set this if your provider requires a specific authentication method and the default doesn't work. List of allowed redirect URI patterns for MCP clients. Patterns support wildcards (e.g., `"http://localhost:*"`, `"https://*.example.com/*"`). - `None` (default): All redirect URIs allowed (for MCP/DCR compatibility) - Empty list `[]`: No redirect URIs allowed - Custom list: Only matching patterns allowed These patterns apply to MCP client loopback redirects, NOT the upstream OAuth app redirect URI. List of all possible valid scopes for the OAuth provider. These are advertised to clients through the `/.well-known` endpoints. Defaults to `required_scopes` from your TokenVerifier if not specified. Additional parameters to forward to the upstream authorization endpoint. Useful for provider-specific parameters that aren't part of the standard OAuth2 flow. For example, Auth0 requires an `audience` parameter to issue JWT tokens: ```python extra_authorize_params={"audience": "https://api.example.com"} ``` These parameters are added to every authorization request sent to the upstream provider. Additional parameters to forward to the upstream token endpoint during code exchange and token refresh. Useful for provider-specific requirements during token operations. For example, some providers require additional context during token exchange: ```python extra_token_params={"audience": "https://api.example.com"} ``` These parameters are included in all token requests to the upstream provider. Storage backend for persisting OAuth client registrations and encrypted upstream tokens. By default, clients are automatically persisted to disk in `~/.config/fastmcp/oauth-proxy-clients/`, allowing them to survive server restarts as long as the filesystem remains accessible. This means MCP clients only need to register once and can reconnect seamlessly after your server restarts. For production deployments with multiple servers or cloud deployments, see [Storage Backends](/servers/storage-backends) for options including Redis, DynamoDB, and custom implementations. For production token persistence, use this with `jwt_signing_key` and `token_encryption_key` - all three work together to ensure tokens survive restarts. See [OAuth Token Security](/deployment/http#oauth-token-security). Testing with in-memory storage: ```python from key_value.aio.stores.memory import MemoryStore # Use in-memory storage for testing (clients lost on restart) auth = OAuthProxy(..., client_storage=MemoryStore()) ``` Production with Redis for distributed deployments: ```python from key_value.aio.stores.redis import RedisStore import os auth = OAuthProxy( ..., jwt_signing_key=os.environ["JWT_SIGNING_KEY"], token_encryption_key=os.environ["TOKEN_ENCRYPTION_KEY"], client_storage=RedisStore(host="redis.example.com", port=6379) ) ``` Secret used to sign FastMCP JWT tokens issued to clients. Accepts any string or bytes - will be derived into a proper 32-byte cryptographic key using HKDF. **Default behavior (None):** Keys are ephemeral (random salt at startup). Tokens become invalid on server restart, triggering client re-authentication. This is fine for development and testing. **Production:** Provide an explicit secret (e.g., from environment variable). Works with `token_encryption_key` and `client_storage` to ensure tokens survive restarts - all three parameters are required for production token persistence. ```python import os auth = OAuthProxy( ..., jwt_signing_key=os.environ["JWT_SIGNING_KEY"], # Any string! token_encryption_key=os.environ["TOKEN_ENCRYPTION_KEY"], client_storage=RedisStore(...) # Persistent storage ) ``` See [HTTP Deployment - OAuth Token Security](/deployment/http#oauth-token-security) for complete production setup. Secret used to encrypt upstream tokens at rest in `client_storage`. Accepts any string or bytes - will be derived into a proper 32-byte cryptographic key using HKDF. **Default behavior (None):** Like `jwt_signing_key`, this is ephemeral. However, without a valid JWT signing key, encrypted tokens are useless anyway (JWT validation fails first). **Production:** Provide an explicit secret distinct from `jwt_signing_key`. Works with `jwt_signing_key` and persistent `client_storage` - all three are required for production token persistence. ```python # Use different secrets for each key jwt_signing_key="my-jwt-secret-v1" token_encryption_key="my-encryption-secret-v1" ``` See [HTTP Deployment - OAuth Token Security](/deployment/http#oauth-token-security). Whether to require user consent before authorizing MCP clients. When enabled (default), users see a consent screen that displays which client is requesting access, preventing [confused deputy attacks](https://modelcontextprotocol.io/specification/2025-06-18/basic/security_best_practices#confused-deputy-problem) by ensuring users explicitly approve new clients. **Default behavior (True):** Users see a consent screen on first authorization. Consent choices are remembered via signed cookies, so users only need to approve each client once. This protects against malicious clients impersonating the user. **Disabling consent (False):** Authorization proceeds directly to the upstream provider without user confirmation. Only use this for local development or testing environments where the security trade-off is acceptable. ```python # Development/testing only - skip consent screen auth = OAuthProxy( ..., require_authorization_consent=False # ⚠️ Security warning: only for local/testing ) ``` Disabling consent removes an important security layer. Only disable for local development or testing environments where you fully control all connecting clients. ### Using Built-in Providers FastMCP includes pre-configured providers for common services: ```python from fastmcp.server.auth.providers.github import GitHubProvider auth = GitHubProvider( client_id="your-github-app-id", client_secret="your-github-app-secret", base_url="https://your-server.com" ) mcp = FastMCP(name="My Server", auth=auth) ``` Available providers include `GitHubProvider`, `GoogleProvider`, and others. These handle token verification automatically. ### Token Verification The OAuth proxy requires a compatible `TokenVerifier` to validate tokens from your provider. Different providers use different token formats: - **JWT tokens** (Google, Azure): Use `JWTVerifier` with the provider's JWKS endpoint - **Opaque tokens with RFC 7662 introspection** (Auth0, Okta, WorkOS): Use `IntrospectionTokenVerifier` - **Opaque tokens (provider-specific)** (GitHub, Discord): Use provider-specific verifiers like `GitHubTokenVerifier` See the [Token Verification guide](/servers/auth/token-verification) for detailed setup instructions for your provider. ### Scope Configuration OAuth scopes control what permissions your application requests from users. They're configured through your `TokenVerifier` (required for the OAuth proxy to validate tokens from your provider). Set `required_scopes` to automatically request the permissions your application needs: ```python JWTVerifier(..., required_scopes = ["read:user", "write:data"]) ``` Dynamic clients created by the proxy will automatically include these scopes in their authorization requests. See the [Token Verification](#token-verification) section below for detailed setup. ### Custom Parameters Some OAuth providers require additional parameters beyond the standard OAuth2 flow. Use `extra_authorize_params` and `extra_token_params` to pass provider-specific requirements. For example, Auth0 requires an `audience` parameter to issue JWT tokens instead of opaque tokens: ```python auth = OAuthProxy( upstream_authorization_endpoint="https://your-domain.auth0.com/authorize", upstream_token_endpoint="https://your-domain.auth0.com/oauth/token", upstream_client_id="your-auth0-client-id", upstream_client_secret="your-auth0-client-secret", # Auth0-specific audience parameter extra_authorize_params={"audience": "https://your-api-identifier.com"}, extra_token_params={"audience": "https://your-api-identifier.com"}, token_verifier=JWTVerifier( jwks_uri="https://your-domain.auth0.com/.well-known/jwks.json", issuer="https://your-domain.auth0.com/", audience="https://your-api-identifier.com" ), base_url="https://your-server.com" ) ``` The proxy also automatically forwards RFC 8707 `resource` parameters from MCP clients to upstream providers that support them. ## OAuth Flow ```mermaid sequenceDiagram participant Client as MCP Client
(localhost:random) participant User as User participant Proxy as FastMCP OAuth Proxy
(server:8000) participant Provider as OAuth Provider
(GitHub, etc.) Note over Client, Proxy: Dynamic Registration (Local) Client->>Proxy: 1. POST /register
redirect_uri: localhost:54321/callback Proxy-->>Client: 2. Returns fixed upstream credentials Note over Client, User: Authorization with User Consent Client->>Proxy: 3. GET /authorize
redirect_uri=localhost:54321/callback
code_challenge=CLIENT_CHALLENGE Note over Proxy: Store transaction with client PKCE
Generate proxy PKCE pair Proxy->>User: 4. Show consent page
(client details, redirect URI, scopes) User->>Proxy: 5. Approve/deny consent Proxy->>Provider: 6. Redirect to provider
redirect_uri=server:8000/auth/callback
code_challenge=PROXY_CHALLENGE Note over Provider, Proxy: Provider Callback Provider->>Proxy: 7. GET /auth/callback
with authorization code Proxy->>Provider: 8. Exchange code for tokens
code_verifier=PROXY_VERIFIER Provider-->>Proxy: 9. Access & refresh tokens Note over Proxy, Client: Client Callback Forwarding Proxy->>Client: 10. Redirect to localhost:54321/callback
with new authorization code Note over Client, Proxy: Token Exchange Client->>Proxy: 11. POST /token with code
code_verifier=CLIENT_VERIFIER Proxy-->>Client: 12. Returns stored provider tokens ``` The flow diagram above illustrates the complete OAuth proxy pattern. Let's understand each phase: ### Registration Phase When an MCP client calls `/register` with its dynamic callback URL, the proxy responds with your pre-configured upstream credentials. The client stores these credentials believing it has registered a new app. Meanwhile, the proxy records the client's callback URL for later use. ### Authorization Phase The client initiates OAuth by redirecting to the proxy's `/authorize` endpoint. The proxy: 1. Stores the client's transaction with its PKCE challenge 2. Generates its own PKCE parameters for upstream security 3. Shows the user a consent page with the client's details, redirect URI, and requested scopes 4. If the user approves (or the client was previously approved), redirects to the upstream provider using the fixed callback URL This dual-PKCE approach maintains end-to-end security at both the client-to-proxy and proxy-to-provider layers. The consent step protects against confused deputy attacks by ensuring you explicitly approve each client before it can complete authorization. ### Callback Phase After user authorization, the provider redirects back to the proxy's fixed callback URL. The proxy: 1. Exchanges the authorization code for tokens with the provider 2. Stores these tokens temporarily 3. Generates a new authorization code for the client 4. Redirects to the client's original dynamic callback URL ### Token Exchange Phase Finally, the client exchanges its authorization code with the proxy to receive the provider's tokens. The proxy validates the client's PKCE verifier before returning the stored tokens. This entire flow is transparent to the MCP client—it experiences a standard OAuth flow with dynamic registration, unaware that a proxy is managing the complexity behind the scenes. ### Token Architecture The OAuth proxy implements a **token factory pattern**: instead of directly forwarding tokens from the upstream OAuth provider, it issues its own JWT tokens to MCP clients. This maintains proper OAuth 2.0 token audience boundaries and enables better security controls. **How it works:** When an MCP client completes authorization, the proxy: 1. **Receives upstream tokens** from the OAuth provider (GitHub, Google, etc.) 2. **Encrypts and stores** these tokens using Fernet encryption (AES-128-CBC + HMAC-SHA256) 3. **Issues FastMCP JWT tokens** to the client, signed with HS256 The FastMCP JWT contains minimal claims: issuer, audience, client ID, scopes, expiration, and a unique token identifier (JTI). The JTI acts as a reference linking to the encrypted upstream token. **Token validation:** When a client makes an MCP request with its FastMCP token: 1. **FastMCP validates the JWT** signature, expiration, issuer, and audience 2. **Looks up the upstream token** using the JTI from the validated JWT 3. **Decrypts and validates** the upstream token with the provider This two-tier validation ensures that FastMCP tokens can only be used with this server (via audience validation) while maintaining full upstream token security. **Token expiry alignment:** FastMCP token lifetimes match the upstream token lifetimes. When the upstream token expires, the FastMCP token also expires, maintaining consistent security boundaries. **Refresh tokens:** The proxy issues its own refresh tokens that map to upstream refresh tokens. When a client uses a FastMCP refresh token, the proxy refreshes the upstream token and issues a new FastMCP access token. **Key and storage configuration:** The token architecture requires cryptographic keys for JWT signing and token encryption. By default, these keys are ephemeral—generated from a random salt at startup and not persisted. This means tokens become invalid on server restart, requiring clients to re-authenticate. For development and testing, this is acceptable. For production, configure three parameters together: `jwt_signing_key` (for signing FastMCP JWTs), `token_encryption_key` (for encrypting upstream tokens at rest), and persistent `client_storage` (for storing encrypted tokens). All three are required for tokens to survive server restarts. The keys accept any secret string and derive proper cryptographic keys using HKDF. See [OAuth Token Security](/deployment/http#oauth-token-security) for complete production setup. ### PKCE Forwarding The OAuth proxy automatically handles PKCE (Proof Key for Code Exchange) when working with providers that support or require it. The proxy generates its own PKCE parameters to send upstream while separately validating the client's PKCE, ensuring end-to-end security at both layers. This is enabled by default via the `forward_pkce` parameter and works seamlessly with providers like Google, Azure AD, and GitHub. Only disable it for legacy providers that don't support PKCE: ```python # Disable PKCE forwarding only if upstream doesn't support it auth = OAuthProxy( ..., forward_pkce=False # Default is True ) ``` ### Redirect URI Validation While the OAuth proxy accepts all redirect URIs by default (for DCR compatibility), you can restrict which clients can connect by specifying allowed patterns: ```python # Allow only localhost clients (common for development) auth = OAuthProxy( # ... other parameters ... allowed_client_redirect_uris=[ "http://localhost:*", "http://127.0.0.1:*" ] ) # Allow specific known clients auth = OAuthProxy( # ... other parameters ... allowed_client_redirect_uris=[ "http://localhost:*", "https://claude.ai/api/mcp/auth_callback", "https://*.mycompany.com/auth/*" # Wildcard patterns supported ] ) ``` Check your server logs for "Client registered with redirect_uri" messages to identify what URLs your clients use. ## Security ### Confused Deputy Attacks A confused deputy attack allows a malicious client to steal your authorization by tricking you into granting it access under your identity. The OAuth proxy works by bridging DCR clients to traditional auth providers, which means that multiple MCP clients connect through a single upstream OAuth application. An attacker can exploit this shared application by registering a malicious client with their own redirect URI, then sending you an authorization link. When you click it, your browser goes through the OAuth flow—but since you may have already authorized this OAuth app before, the provider might auto-approve the request. The authorization code then gets sent to the attacker's redirect URI instead of a legitimate client, giving them access under your credentials. #### Mitigation FastMCP's OAuth proxy requires you to explicitly consent whenever any new or unrecognized client attempts to connect to your server. Before any authorization happens, you see a consent page showing the client's details, redirect URI, and requested scopes. This gives you the opportunity to review and deny suspicious requests. Once you approve a client, it's remembered so you don't see the consent page again for that client. The consent mechanism is implemented with CSRF tokens and cryptographically signed cookies to prevent tampering. ![](/assets/images/oauth-proxy-consent-screen.png) The consent page automatically displays your server's name, icon, and website URL, if available. These visual identifiers help users confirm they're authorizing the correct server. **Learn more:** - [MCP Security Best Practices](https://modelcontextprotocol.io/specification/2025-06-18/basic/security_best_practices#confused-deputy-problem) - Official specification guidance - [Confused Deputy Attacks Explained](https://den.dev/blog/mcp-confused-deputy-api-management/) - Detailed walkthrough by Den Delimarsky ## Environment Configuration For production deployments, configure the OAuth proxy through environment variables instead of hardcoding credentials: ```bash # Specify the provider implementation export FASTMCP_SERVER_AUTH=fastmcp.server.auth.providers.github.GitHubProvider # Provider-specific credentials export FASTMCP_SERVER_AUTH_GITHUB_CLIENT_ID="Ov23li..." export FASTMCP_SERVER_AUTH_GITHUB_CLIENT_SECRET="abc123..." export FASTMCP_SERVER_AUTH_GITHUB_BASE_URL="https://your-production-server.com" ``` With environment configuration, your server code simplifies to: ```python from fastmcp import FastMCP # Authentication automatically configured from environment mcp = FastMCP(name="My Server") @mcp.tool def protected_tool(data: str) -> str: """This tool is now protected by OAuth.""" return f"Processed: {data}" if __name__ == "__main__": mcp.run(transport="http", port=8000) ```