Cross-domain-cookies - a maybe new idea

Question

My buddy Carsten Lau came along with an interesting idea on how to read cross-domain-cookies.

Situation: You want to read a cookie from domain "A" that was set on domain "B". Idea: From the client on domain "A", you execute a get-request to a dynamic resource on domain "B" – f.e. an image or javascript, which on the server "B" is in fact a programming language capable of reading cookies like PHP, Java etc. With that request, you send an unique identifier like a session id. So the code on the client which looks at a site on domain "A" could look like this:

<img src="www.domainB.com/?getCookie.php?sessionID=1234"> 

Now comes the funny part, server B reads on server-side the cookie set by domain "B" and writes the result with the provided session-id either in a DB accessible by domain "A" or returns a response which contains the cookie information to the client on domain "A" which then sends it via AJAX to server "A".

I am pretty sure there is a flaw we didn't find yet. I personally believe server "B" will not be able to read cookie informations because the client-browsers URL points to domain "A", but of course the "getCookie"-request explained above points to "B".

Please tell us what you think about it, why it works or why it can't work. A small proof of concept was, to my big surprise, successful.

Answer 1

This is normal, because you have control of the 2 domains. This is how most websites achieve cross domain single-sign-on by the way. But if you do not have control of the second domain you cannot read cookies from it.