javascript - Are AJAX calls to a sub-domain considered Cross Site Scripting?

Question

I have Server A (www.example.com) sending information to Server B. I can only have HTML / JS on Server A (and have to do the "crunching" on Server B) so I'm trying to send form data via AJAX (trying to avoid a form post to Server B - don't ask).

Obviously doing an AJAX call cross-domain is considered XSS and a big no-no, but if I were to put Server B in a subdomain (sub.example.com), would that be considered okay? How are cross-domain errors detected? Does the browser look up DNS records? IP address?

Thanks in advance for you help.

Answer 1

Sub-domains are considered different and will fail the Same Origin Policy unless both sub-domains declare the same document.domain DOM property (and even then, different browsers behave differently).