I have paste some HTML code to a Edouard using MessengerFX, a popular web Windows Live Messenger client based on AJAX, and – surprise, the code has been interpreted. Oh?! A XSS vulnerability ? Yes, and such a big one!
MessengerFX is popular and growing, such a flaw can be very dangerous for a lot of people. I have send a mail to the development team and I hope they will correct their application soon… Because the fix is as simple as a htmlspecialchars() call. MessengerFX users, don’t use it anymore and try Meebo or the official Microsoft WLM web based client. Web developers, never trust the user-submitted data and always escape thos inputs!!
Edit october 6 2008 : the problem is now corrected.