Ubuntu Security Notice 597-1 – Timo Juhani Lindfors discovered that the OpenSSH client, when port forwarding was requested, would listen on any available address family. A local attacker could exploit this flaw on systems with IPv6 enabled to hijack connections, including X11 forwards.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/65092/USN-597-1.txt
Source: https://packetstormsecurity.com/files/65092/Ubuntu-Security-Notice-597-1.html