Handling CORS

Configuration

Since Socket.IO v3, you need to explicitly enable Cross-Origin Resource Sharing (CORS).

import { createServer } from "http";
import { Server } from "socket.io";

const httpServer = createServer();
const io = new Server(httpServer, {
  cors: {
    origin: "https://example.com"
  }
});

All options will be forwarded to the cors package. The complete list of options can be found here.

Example with cookies (withCredentials) and additional headers:

// server-side
const io = new Server(httpServer, {
  cors: {
    origin: "https://example.com",
    allowedHeaders: ["my-custom-header"],
    credentials: true
  }
});

// client-side
import { io } from "socket.io-client";
const socket = io("https://api.example.com", {
  withCredentials: true,
  extraHeaders: {
    "my-custom-header": "abcd"
  }
});

Note: this also applies to localhost if your web application and your server are not served from the same port

const io = new Server(httpServer, {
  cors: {
    origin: "http://localhost:8080"
  }
});

httpServer.listen(3000);

You can disallow all cross-origin requests with the allowRequest option:

const io = new Server(httpServer, {
  allowRequest: (req, callback) => {
    const noOriginHeader = req.headers.origin === undefined;
    callback(null, noOriginHeader);
  }
});

Troubleshooting

CORS header ‘Access-Control-Allow-Origin’ missing

Full error message:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at .../socket.io/?EIO=4&transport=polling&t=NMnp2WI. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).

If you have properly configured your server (see above), this could mean that your browser wasn't able to reach the Socket.IO server.

The following command:

curl "https://api.example.com/socket.io/?EIO=4&transport=polling"

should return something like:

0{"sid":"Lbo5JLzTotvW3g2LAAAA","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":20000}

If that's not the case, please check that your server is listening and is actually reachable on the given port.

Credential is not supported if the CORS header ‘Access-Control-Allow-Origin’ is ‘*’

Full error message:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at ‘.../socket.io/?EIO=4&transport=polling&t=NvQfU77’. (Reason: Credential is not supported if the CORS header ‘Access-Control-Allow-Origin’ is ‘*’)

You can't set withCredentials to true with origin: *, you need to use a specific origin:

import { createServer } from "http";
import { Server } from "socket.io";

const httpServer = createServer();
const io = new Server(httpServer, {
  cors: {
    origin: "https://my-frontend.com",
    // or with an array of origins
    // origin: ["https://my-frontend.com", "https://my-other-frontend.com", "http://localhost:3000"],
    credentials: true
  }
});

Expected ‘true’ in CORS header ‘Access-Control-Allow-Credentials’

Full error message:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at .../socket.io/?EIO=4&transport=polling&t=NvQny19. (Reason: expected ‘true’ in CORS header ‘Access-Control-Allow-Credentials’)

In that case, withCredentials is set to true on the client, but the server is missing the credentials attribute in the cors option. See the example above.