cors-gate

Connect-compatible middleware to selectively reject requests based on CORS rules.

This lets you implement an elegant alternative to CSRF tokens if you only need to support modern browsers. For more information, see our blog post.

Install

Run this in your project:

$ npm install cors-gate

Test

$ npm test

Usage

const express = require('express');
const cors = require('cors');
const corsGate = require('cors-gate');

const app = express();

app.use(cors({
  origin: ['https://app.mixmax.com', 'https://other-app.mixmax.com'],
  credentials: true
}));

// prevent cross-origin requests from domains not permitted by the preceeding cors rules
app.use(corsGate({
  // require an Origin header, and reject request if missing
  strict: true,
  // permit GET and HEAD requests, even without an Origin header
  allowSafe: true,
  // the origin of the server
  origin: 'https://api.mixmax.com'
}));

// add a new contact
app.post('/api/contacts', function(req, res) {
  // ...
  res.status(200).json({id: id});
});

Alternative failure handling

By default, cors-gate will return 403 Unauthorized to any requests that aren't permitted by the specified options.

The failure option offers a means to change this behavior. This way, unauthorized cross-origin requests can be permitted in a restricted manner - perhaps by requiring an explicit authentication mechanism rather than cookie-based authentication to prevent cross-site scripting. As such, cors-gate can serve as a CSRF mechanism without the need for a token, while still allowing limited forms of third-party cross-origin API requests.

app.use(corsGate({
  origin: 'https://api.mixmax.com',
  failure: function(req, res, next) {
    // requests from other origins will have this flag set.
    req.requireExplicitAuthentication = true;
    next();
  }
}));

Firefox and the Origin header

Firefox does not set the Origin header on same-origin requests (see also csrf-request-tester) for same-origin requests, as of version 53. The corsGate.originFallbackToReferrer middleware will, if the Origin header is missing, fill it with the origin part of the Referer. This middleware thus enables verification of the Origin for same-origin requests.

Additionally, no browser sends the Origin header when sending a GET request to load an image. We could simply allow all GET requests - GET requests are safe, per HTTP - but we'd rather reject unauthorized cross-origin GET requests wholesale.

At present, Chrome and Safari do not support the strict-origin Referrer-Policy, so we can only patch the Origin from the Referer on Firefox. In patching it, however, we can reject unauthorized cross-origin GET requests from images, and once Chrome and Safari support strict-origin, we'll be able to do so on all three platforms.

In order to actually reject these requests, however, the patched Origin data must be visible to the cors middleware. This middleware is distinct because it must appear before cors and corsGate to perform all the described tasks.

app.use(corsGate.originFallbackToReferrer());
app.use(cors({ ... }));
app.use(corsGate({ ... }));

Language ports

• Go: Roemerb/corsgate

License

The MIT License.