OpenSSH before 5.1 sets the SO_REUSEADDR socket option when the X11UseLocalhost configuration setting is disabled, which allows local users on some platforms to hijack the X11 forwarding port via a bind to a single IP address, as demonstrated on the HP-UX platform.
http://secunia.com/advisories/31179
https://exchange.xforce.ibmcloud.com/vulnerabilities/43940