That's the basic principle of XSS. A few years ago an XSS epidemic broke out where dozens of major websites were found to be vulnerable to cookie theft. Attacke

That's the basic principle of XSS. A few years ago an XSS epidemic broke out where dozens of major websites were found to be vulnerable to cookie theft. Attackers could make a single page with dozens of sneaky iframes, one per vulnerability. Usually the contents of the cookie allows you to continue a user's session, though there can be all kinds of stuff idiotically stored directly in the cookie, as can be seen in this prime example.