Why does the preflight OPTIONS request of an authenticated CORS request work in Chrome but not Firefox?

Question

I am writing a JavaScript client to be included on 3rd party sites (think Facebook Like button). It needs to retrieve information from an API that requires basic HTTP authentication. The simplified setup looks like this:

A 3rd party site includes this snippet on their page:

<script 
async="true"
id="web-dev-widget"
data-public-key="pUbl1c_ap1k3y"
src="http://web.dev/widget.js">
</script>

widget.js calls the API:

var el = document.getElementById('web-dev-widget'),
    user = 'token',
    pass = el.getAttribute('data-public-key'),
    url = 'https://api.dev/',
    httpRequest = new XMLHttpRequest(),
    handler = function() {
      if (httpRequest.readyState === 4) {
        if (httpRequest.status === 200) {
          console.log(httpRequest.responseText);
        } else {
          console.log('There was a problem with the request.', httpRequest);
        }
      }
    };

httpRequest.open('GET', url, true, user, pass);
httpRequest.onreadystatechange = handler;
httpRequest.withCredentials = true;
httpRequest.send();

The API has been configured to respond with appropriate headers:

Header set Access-Control-Allow-Credentials: true
Header set Access-Control-Allow-Methods: "GET, OPTIONS"
Header set Access-Control-Allow-Headers: "origin, authorization, accept"
SetEnvIf Origin "http(s)?://(.+?\.[a-z]{3})$" AccessControlAllowOrigin=$0
Header set Access-Control-Allow-Origin %{AccessControlAllowOrigin}e env=AccessControlAllowOrigin

Note that the Access-Control-Allow-Origin is set to the Origin instead of using a wildcard because I am sending a credentialed request (withCredentials).

Everything is now in place to make an asynchronous cross-domain authenticated request, and it works great in Chrome 25 on OS X 10.8.2. In Dev Tools, I can see the network request for the OPTIONS request before the GET request, and the response comes back as expected.

When testing in Firefox 19, no network requests appear in Firebug to the API, and this error is logged in the console: NS_ERROR_DOM_BAD_URI: Access to restricted URI denied

After much digging, I found that Gecko doesn't allow the username and password to be directly in a cross-site URI according to the comments. I assumed this was from using the optional user and password params to open() so I tried the other method of making authenticated requests which is to Base64 encode the credentials and send in an Authorization header:

// Base64 from http://www.webtoolkit.info/javascript-base64.html
auth = "Basic " + Base64.encode(user + ":" + pass);

...
// after open() and before send()
httpRequest.setRequestHeader('Authorization', auth);

This results in a 401 Unauthorized response to the OPTIONS request which lead to Google searches like, "Why does this work in Chrome and not Firefox!?" That's when I knew I was in trouble.

Why does it work in Chrome and not Firefox? How can I get the OPTIONS request to send and respond consistently?

Answer 1

Why does it work in Chrome and not Firefox?

The W3 spec for CORS preflight requests clearly states that user credentials should be excluded. There is a bug in Chrome and WebKit where OPTIONS requests returning a status of 401 still send the subsequent request.

Firefox has a related bug filed that ends with a link to the W3 public webapps mailing list asking for the CORS spec to be changed to allow authentication headers to be sent on the OPTIONS request at the benefit of IIS users. Basically, they are waiting for those servers to be obsoleted.

How can I get the OPTIONS request to send and respond consistently?

Simply have the server (API in this example) respond to OPTIONS requests without requiring authentication.

Kinvey did a good job expanding on this while also linking to an issue of the Twitter API outlining the catch-22 problem of this exact scenario interestingly a couple weeks before any of the browser issues were filed.