I want your help on getting more concrete information on a type of vulnerability that I remember vaguely.
I vaguely remember hearing about a year ago that if you set up a webapp on a subdomain, and that webapp gets compromised, then your main webapp (on the root domain) could be compromised as well, because (and this is the part that's vague in my memory) the browser has some assumptions about the two different webapps being the same entity because they're from the same root domain.
I heard that this why Facebook originally hosted their developer forums on a different domain instead of a subdomain of facebook.com; because the forum app was far, far less extensively audited for security problems than Facebook itself, and they didn't want any security holes in the forums to allow an attacker to gain access to Facebook itself.
(To clarify, these attacks are based on the browser, not on some kind of connection between the two servers.)
My question: Do you have the specifics of what those vulnerabilities are and how I could protect against them so I'd be confident that an attacker getting access to one webapp couldn't get access to a different webapp on the same root domain?