Extract client authentication into reusable middleware

src/server/auth/clients.ts

@@ -12,7 +12,7 @@ export interface OAuthRegisteredClientsStore {
   /**
    * Registers a new client with the server. The client ID and secret will be automatically generated by the library. A modified version of the client information can be returned to reflect specific values enforced by the server.
    * 
-   * NOTE: Implements must ensure that client secrets, if present, are expired in accordance with the `client_secret_expires_at` field.
+   * NOTE: Implementations must ensure that client secrets, if present, are expired in accordance with the `client_secret_expires_at` field.
    * 
    * If unimplemented, dynamic client registration is unsupported.
    */

src/server/auth/handlers/token.ts

@@ -3,14 +3,13 @@ import express, { RequestHandler } from "express";
 import { OAuthServerProvider } from "../provider.js";
 import cors from "cors";
 import { verifyChallenge } from "pkce-challenge";
+import { authenticateClient } from "../middleware/clientAuth.js";
 
 export type TokenHandlerOptions = {
   provider: OAuthServerProvider;
 };
 
 const TokenRequestSchema = z.object({
-  client_id: z.string(),
-  client_secret: z.string().optional(),
   grant_type: z.string(),
 });
 
@@ -32,10 +31,13 @@ export function tokenHandler({ provider }: TokenHandlerOptions): RequestHandler
   // Configure CORS to allow any origin, to make accessible to web-based MCP clients
   router.use(cors());
 
+  // Authenticate and extract client details
+  router.use(authenticateClient({ clientsStore: provider.clientsStore }));
+
   router.post("/", async (req, res) => {
-    let client_id, client_secret, grant_type;
+    let grant_type;
     try {
-      ({ client_id, client_secret, grant_type } = TokenRequestSchema.parse(req.body));
+      ({ grant_type } = TokenRequestSchema.parse(req.body));
     } catch (error) {
       res.status(400).json({
         error: "invalid_request",
@@ -44,24 +46,6 @@ export function tokenHandler({ provider }: TokenHandlerOptions): RequestHandler
       return;
     }
 
-    const client = await provider.clientsStore.getClient(client_id);
-    if (!client) {
-      // TODO: Return 401 with WWW-Authenticate if Authorization header was used
-      res.status(400).json({
-        error: "invalid_client",
-        error_description: "Invalid client_id",
-      });
-      return;
-    }
-
-    if (client.client_secret !== client_secret) {
-      res.status(400).json({
-        error: "invalid_client",
-        error_description: "Invalid client_secret",
-      });
-      return;
-    }
-
     switch (grant_type) {
       case "authorization_code": {
         let grant;

src/server/auth/middleware/clientAuth.ts

@@ -0,0 +1,65 @@
+import { z } from "zod";
+import { RequestHandler } from "express";
+import { OAuthRegisteredClientsStore } from "../clients.js";
+import { OAuthClientInformationFull } from "../../../shared/auth.js";
+
+export type ClientAuthenticationMiddlewareOptions = {
+  /**
+   * A store used to read information about registered OAuth clients.
+   */
+  clientsStore: OAuthRegisteredClientsStore;
+}
+
+const ClientAuthenticatedRequestSchema = z.object({
+  client_id: z.string(),
+  client_secret: z.string().optional(),
+})
+
+declare global {
+  namespace Express {
+    interface Request {
+      /**
+       * The authenticated client for this request, if the `authenticateClient` middleware was used.
+       */
+      client?: OAuthClientInformationFull;
+    }
+  }
+}
+
+export function authenticateClient({ clientsStore }: ClientAuthenticationMiddlewareOptions): RequestHandler {
+  return async (req, res, next) => {
+    let client_id, client_secret;
+    try {
+      const result = ClientAuthenticatedRequestSchema.parse(req.body);
+      client_id = result.client_id;
+      client_secret = result.client_secret;
+    } catch (error) {
+      res.status(400).json({
+        error: "invalid_request",
+        error_description: String(error),
+      });
+      return;
+    }
+
+    const client = await clientsStore.getClient(client_id);
+    if (!client) {
+      // TODO: Return 401 with WWW-Authenticate if Authorization header was used
+      res.status(400).json({
+        error: "invalid_client",
+        error_description: "Invalid client_id",
+      });
+      return;
+    }
+
+    if (client.client_secret !== client_secret) {
+      res.status(400).json({
+        error: "invalid_client",
+        error_description: "Invalid client_secret",
+      });
+      return;
+    }
+
+    req.client = client;
+    next();
+  }
+}

src/server/auth/provider.ts

@@ -33,6 +33,7 @@ export interface OAuthServerProvider {
   /**
    * Returns the `codeChallenge` that was used when the indicated authorization began.
    */
+  // TODO: Add `client` to this and below methods
   challengeForAuthorizationCode(authorizationCode: string): Promise<string>;
 
   /**