Implement DNS Rebinding Protections per spec

README.md

@@ -444,7 +444,11 @@ app.post('/mcp', async (req, res) => {
       onsessioninitialized: (sessionId) => {
         // Store the transport by session ID
         transports[sessionId] = transport;
-      }
+      },
+      // DNS rebinding protection is disabled by default for backwards compatibility. If you are running this server
+      // locally, make sure to set:
+      // enableDnsRebindingProtection: true,
+      // allowedHosts: ['127.0.0.1'],
     });
 
     // Clean up transport when closed
@@ -596,6 +600,22 @@ This stateless approach is useful for:
 - RESTful scenarios where each request is independent
 - Horizontally scaled deployments without shared session state
 
+#### DNS Rebinding Protection
+
+The Streamable HTTP transport includes DNS rebinding protection to prevent security vulnerabilities. By default, this protection is **disabled** for backwards compatibility.
+
+**Important**: If you are running this server locally, enable DNS rebinding protection:
+
+```typescript
+const transport = new StreamableHTTPServerTransport({
+  sessionIdGenerator: () => randomUUID(),
+  enableDnsRebindingProtection: true,
+
+  allowedHosts: ['127.0.0.1', ...],
+  allowedOrigins: ['https://yourdomain.com', 'https://www.yourdomain.com']
+});
+```
+
 ### Testing and Debugging
 
 To test your server, you can use the [MCP Inspector](https://github.com/modelcontextprotocol/inspector). See its README for more information.

src/server/sse.test.ts

@@ -453,4 +453,264 @@ describe('SSEServerTransport', () => {
         expect.stringContaining(`data: /messages?sessionId=${transport.sessionId}`));
     });
   });
-});
+
+  describe('DNS rebinding protection', () => {
+    beforeEach(() => {
+      jest.clearAllMocks();
+    });
+
+    describe('Host header validation', () => {
+      it('should accept requests with allowed host headers', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes, {
+          allowedHosts: ['localhost:3000', 'example.com'],
+          enableDnsRebindingProtection: true,
+        });
+        await transport.start();
+
+        const mockReq = createMockRequest({
+          headers: {
+            host: 'localhost:3000',
+            'content-type': 'application/json',
+          }
+        });
+        const mockHandleRes = createMockResponse();
+
+        await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes.writeHead).toHaveBeenCalledWith(202);
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Accepted');
+      });
+
+      it('should reject requests with disallowed host headers', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes, {
+          allowedHosts: ['localhost:3000'],
+          enableDnsRebindingProtection: true,
+        });
+        await transport.start();
+
+        const mockReq = createMockRequest({
+          headers: {
+            host: 'evil.com',
+            'content-type': 'application/json',
+          }
+        });
+        const mockHandleRes = createMockResponse();
+
+        await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes.writeHead).toHaveBeenCalledWith(403);
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Invalid Host header: evil.com');
+      });
+
+      it('should reject requests without host header when allowedHosts is configured', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes, {
+          allowedHosts: ['localhost:3000'],
+          enableDnsRebindingProtection: true,
+        });
+        await transport.start();
+
+        const mockReq = createMockRequest({
+          headers: {
+            'content-type': 'application/json',
+          }
+        });
+        const mockHandleRes = createMockResponse();
+
+        await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes.writeHead).toHaveBeenCalledWith(403);
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Invalid Host header: undefined');
+      });
+    });
+
+    describe('Origin header validation', () => {
+      it('should accept requests with allowed origin headers', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes, {
+          allowedOrigins: ['http://localhost:3000', 'https://example.com'],
+          enableDnsRebindingProtection: true,
+        });
+        await transport.start();
+
+        const mockReq = createMockRequest({
+          headers: {
+            origin: 'http://localhost:3000',
+            'content-type': 'application/json',
+          }
+        });
+        const mockHandleRes = createMockResponse();
+
+        await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes.writeHead).toHaveBeenCalledWith(202);
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Accepted');
+      });
+
+      it('should reject requests with disallowed origin headers', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes, {
+          allowedOrigins: ['http://localhost:3000'],
+          enableDnsRebindingProtection: true,
+        });
+        await transport.start();
+
+        const mockReq = createMockRequest({
+          headers: {
+            origin: 'http://evil.com',
+            'content-type': 'application/json',
+          }
+        });
+        const mockHandleRes = createMockResponse();
+
+        await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes.writeHead).toHaveBeenCalledWith(403);
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Invalid Origin header: http://evil.com');
+      });
+    });
+
+    describe('Content-Type validation', () => {
+      it('should accept requests with application/json content-type', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes);
+        await transport.start();
+
+        const mockReq = createMockRequest({
+          headers: {
+            'content-type': 'application/json',
+          }
+        });
+        const mockHandleRes = createMockResponse();
+
+        await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes.writeHead).toHaveBeenCalledWith(202);
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Accepted');
+      });
+
+      it('should accept requests with application/json with charset', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes);
+        await transport.start();
+
+        const mockReq = createMockRequest({
+          headers: {
+            'content-type': 'application/json; charset=utf-8',
+          }
+        });
+        const mockHandleRes = createMockResponse();
+
+        await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes.writeHead).toHaveBeenCalledWith(202);
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Accepted');
+      });
+
+      it('should reject requests with non-application/json content-type when protection is enabled', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes);
+        await transport.start();
+
+        const mockReq = createMockRequest({
+          headers: {
+            'content-type': 'text/plain',
+          }
+        });
+        const mockHandleRes = createMockResponse();
+
+        await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes.writeHead).toHaveBeenCalledWith(400);
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Error: Unsupported content-type: text/plain');
+      });
+    });
+
+    describe('enableDnsRebindingProtection option', () => {
+      it('should skip all validations when enableDnsRebindingProtection is false', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes, {
+          allowedHosts: ['localhost:3000'],
+          allowedOrigins: ['http://localhost:3000'],
+          enableDnsRebindingProtection: false,
+        });
+        await transport.start();
+
+        const mockReq = createMockRequest({
+          headers: {
+            host: 'evil.com',
+            origin: 'http://evil.com',
+            'content-type': 'text/plain',
+          }
+        });
+        const mockHandleRes = createMockResponse();
+
+        await transport.handlePostMessage(mockReq, mockHandleRes, { jsonrpc: '2.0', method: 'test' });
+
+        // Should pass even with invalid headers because protection is disabled
+        expect(mockHandleRes.writeHead).toHaveBeenCalledWith(400);
+        // The error should be from content-type parsing, not DNS rebinding protection
+        expect(mockHandleRes.end).toHaveBeenCalledWith('Error: Unsupported content-type: text/plain');
+      });
+    });
+
+    describe('Combined validations', () => {
+      it('should validate both host and origin when both are configured', async () => {
+        const mockRes = createMockResponse();
+        const transport = new SSEServerTransport('/messages', mockRes, {
+          allowedHosts: ['localhost:3000'],
+          allowedOrigins: ['http://localhost:3000'],
+          enableDnsRebindingProtection: true,
+        });
+        await transport.start();
+
+        // Valid host, invalid origin
+        const mockReq1 = createMockRequest({
+          headers: {
+            host: 'localhost:3000',
+            origin: 'http://evil.com',
+            'content-type': 'application/json',
+          }
+        });
+        const mockHandleRes1 = createMockResponse();
+
+        await transport.handlePostMessage(mockReq1, mockHandleRes1, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes1.writeHead).toHaveBeenCalledWith(403);
+        expect(mockHandleRes1.end).toHaveBeenCalledWith('Invalid Origin header: http://evil.com');
+
+        // Invalid host, valid origin
+        const mockReq2 = createMockRequest({
+          headers: {
+            host: 'evil.com',
+            origin: 'http://localhost:3000',
+            'content-type': 'application/json',
+          }
+        });
+        const mockHandleRes2 = createMockResponse();
+
+        await transport.handlePostMessage(mockReq2, mockHandleRes2, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes2.writeHead).toHaveBeenCalledWith(403);
+        expect(mockHandleRes2.end).toHaveBeenCalledWith('Invalid Host header: evil.com');
+
+        // Both valid
+        const mockReq3 = createMockRequest({
+          headers: {
+            host: 'localhost:3000',
+            origin: 'http://localhost:3000',
+            'content-type': 'application/json',
+          }
+        });
+        const mockHandleRes3 = createMockResponse();
+
+        await transport.handlePostMessage(mockReq3, mockHandleRes3, { jsonrpc: '2.0', method: 'test' });
+
+        expect(mockHandleRes3.writeHead).toHaveBeenCalledWith(202);
+        expect(mockHandleRes3.end).toHaveBeenCalledWith('Accepted');
+      });
+    });
+  });
+});

src/server/sse.ts

@@ -9,6 +9,29 @@ import { URL } from 'url';
 
 const MAXIMUM_MESSAGE_SIZE = "4mb";
 
+/**
+ * Configuration options for SSEServerTransport.
+ */
+export interface SSEServerTransportOptions {
+  /**
+   * List of allowed host header values for DNS rebinding protection.
+   * If not specified, host validation is disabled.
+   */
+  allowedHosts?: string[];
+
+  /**
+   * List of allowed origin header values for DNS rebinding protection.
+   * If not specified, origin validation is disabled.
+   */
+  allowedOrigins?: string[];
+
+  /**
+   * Enable DNS rebinding protection (requires allowedHosts and/or allowedOrigins to be configured).
+   * Default is false for backwards compatibility.
+   */
+  enableDnsRebindingProtection?: boolean;
+}
+
 /**
  * Server transport for SSE: this will send messages over an SSE connection and receive messages from HTTP POST requests.
  *
@@ -17,6 +40,7 @@ const MAXIMUM_MESSAGE_SIZE = "4mb";
 export class SSEServerTransport implements Transport {
   private _sseResponse?: ServerResponse;
   private _sessionId: string;
+  private _options: SSEServerTransportOptions;
   onclose?: () => void;
   onerror?: (error: Error) => void;
   onmessage?: (message: JSONRPCMessage, extra?: MessageExtraInfo) => void;
@@ -27,8 +51,39 @@ export class SSEServerTransport implements Transport {
   constructor(
     private _endpoint: string,
     private res: ServerResponse,
+    options?: SSEServerTransportOptions,
   ) {
     this._sessionId = randomUUID();
+    this._options = options || {enableDnsRebindingProtection: false};
+  }
+
+  /**
+   * Validates request headers for DNS rebinding protection.
+   * @returns Error message if validation fails, undefined if validation passes.
+   */
+  private validateRequestHeaders(req: IncomingMessage): string | undefined {
+    // Skip validation if protection is not enabled
+    if (!this._options.enableDnsRebindingProtection) {
+      return undefined;
+    }
+
+    // Validate Host header if allowedHosts is configured
+    if (this._options.allowedHosts && this._options.allowedHosts.length > 0) {
+      const hostHeader = req.headers.host;
+      if (!hostHeader || !this._options.allowedHosts.includes(hostHeader)) {
+        return `Invalid Host header: ${hostHeader}`;
+      }
+    }
+
+    // Validate Origin header if allowedOrigins is configured
+    if (this._options.allowedOrigins && this._options.allowedOrigins.length > 0) {
+      const originHeader = req.headers.origin;
+      if (!originHeader || !this._options.allowedOrigins.includes(originHeader)) {
+        return `Invalid Origin header: ${originHeader}`;
+      }
+    }
+
+    return undefined;
   }
 
   /**
@@ -85,6 +140,15 @@ export class SSEServerTransport implements Transport {
       res.writeHead(500).end(message);
       throw new Error(message);
     }
+
+    // Validate request headers for DNS rebinding protection
+    const validationError = this.validateRequestHeaders(req);
+    if (validationError) {
+      res.writeHead(403).end(validationError);
+      this.onerror?.(new Error(validationError));
+      return;
+    }
+
     const authInfo: AuthInfo | undefined = req.auth;
     const requestInfo: RequestInfo = { headers: req.headers };