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.
https://exchange.xforce.ibmcloud.com/vulnerabilities/43940
http://www.vupen.com/english/advisories/2008/2148
http://www.securitytracker.com/id?1020537
http://www.securityfocus.com/bid/30339
http://www.openssh.com/txt/release-5.1