Mozilla has fixed a bug in the way that its Bugzilla Web site and others handled certain errors, which could have been exploited to execute a man-in-the-middle attack against an unsuspecting user.
The bug was related to the way that the sites responded to certain requests from client machines when the clients specify an incorrect HTTP host header. The Bugzilla site holds a wild card SSL certificate that also is valid on Mozilla.org, and as a result when the sites respond to the request with the incorrect header, clients can be redirected to a non-HTTPS site for an error message.
"As a result, a network attacker can divert a client connection bound for any *.mozilla.org site to one of these servers and cause the client to receive an incorrect redirect. This is already a breach of the integrity that SSL is supposed to provide. But what is worse, since the redirect is to http://, the attacker can substitute arbitrary content and thereby perform XSS," Matt McCutchen wrote in an explanation of the certificate problem on Bugzilla.
More on attack scenario at http://threatpost.com/en_us/blogs/mozilla-fixes-site-error-handling-bug-112210