Cross-domain security woes

You're developing an Ajax-based application. You have an application server at example.com which serves up all your JavaScript, HTML and CSS, and a data server at xml.example.com which delivers all the XML data to the application via a hidden IFRAME.

You know that cross-domain security will prevent any JavaScript from accessing the data in the IFRAME. so, you configure the data server to set the security domain of the IFRAME to "example.com" — the common suffix between the two domains — with a small piece of JavaScript:

<script type="text/javascript">
  document.domain="example.com";
</script>

Having done this, you test your application and get a "permission denied" error. What happened?

Depending on your browser, it may not be enough to only set the security domain of the IFRAME. You must set all of the frames and windows to the same domain, too. This is true even if the domain name you're trying to set already matches the domain of the server that's currently serving the page. For example, if you have two frames with pages served from example.com and you use JavaScript to set the security domain of one frame to "example.com" the frames will be unable to communicate.

Older browsers might let you get away with this.