OpenSSH SSH client before 2.3.0 does not properly disable X11 or agent forwarding, which could allow a malicious SSH server to gain access to the X11 display and sniff X11 events, or gain access to the ssh-agent.
http://archives.neohapsis.com/archives/bugtraq/2000-11/0195.html
http://archives.neohapsis.com/archives/bugtraq/2000-11/0217.html
http://lists.suse.com/archives/suse-security-announce/2000-Nov/0004.html
https://exchange.xforce.ibmcloud.com/vulnerabilities/5517