Using Cloudflare Workers to Optimize Auth0 Universal Login

Auth0 is still one of the best ways to add authorization to your app. However, one minor annoyance I have found is that there is no way to force every login to use a single identity provider (i.e. connection) without configuring each client with the connection parameter. So even for an app that only allows users to login with a single social connection (i.e. Google) users will still see the Auth0 login picker by default.

With Cloudflare Workers and an Auth0 custom domain it is easy to fix this issue. After you setup your custom domain, you need to make sure you are proxying the CNAME through Cloudflare.

Next, create a simple Cloudflare Worker with the following code.

export default {
  async fetch(request, env) {
    return await handleRequest(request);
  },
};

const AUTH0_CONNECTION = "my-connection";

async function handleRequest(request) {
  const url = new URL(request.url);
  // Checking path just in case, but this
  // worker should only run on this path
  if (url.pathname === "/authorize") {
    if (url.searchParams.get("connection")) {
      return fetch(request);
    } else {
      url.searchParams.set("connection", AUTH0_CONNECTION);
      const newRequest = new Request(url.toString(), new Request(request));
      return fetch(newRequest);
    }
  }
  return fetch(request);
}

Finally, configure a Route for the Cloudflare worker to run on the /authorize path. The route should look like this: my-cname.example.com/authorize*. Make sure to put the * at the end; otherwise, requests with the query parameters will not be sent to the worker.

When a request comes to the regular /authorize URL to start an OAuth flow, the connection query parameter is added automatically. Every user will skip the Auth0 login page and immediately go to the login page of the specified connection.

One word of caution, I am unsure if Auth0 supports this configuration. It works for me, but Auth0 could make changes that break this at some point.

Hopefully, this helps solve a minor annoyance you might have with Auth0.