Thanks to Luigi Auriemma a new bug was traced down in a fully patched QuickTime version 7.3.1, running on Windows and Mac OS X.
The problem is a buffer-overflow which happens during the filling of the LCD-like screen containing info about the status of the connection. For exploiting this vulnerability is only needed that a user follows a rtsp:// link, if the port 554 of the server is closed Quicktime will automatically change the transport and will try the HTTP protocol on port 80, the 404 error message of the server (other error numbers are valid too) will be visualized in the LCD-like screen.
During Luigi Auriemma’s tests he has been able to fully overwrite the return address anyway note that the visible effects of the vulnerability could change during the usage of the debugger (in attaching mode it's everything ok).
For all users, it is recommended updating QuickTime 7.3.1.
The full information can be found at http://www.milw0rm.com/exploits/4885