Vulnerability Discussion

When X11 forwarding is enabled, there may be additional exposure to the server and client displays if the sshd proxy display is configured to listen on the wildcard address. By default, sshd binds the forwarding server to the loopback address and sets the hostname part of the "DISPLAY" environment variable to localhost. This prevents remote hosts from connecting to the proxy display.

Check

Verify the SSH daemon prevents remote hosts from connecting to the proxy display with the following command:

$ sshd -T | grep x11uselocalhost

x11uselocalhost yes

If the value is returned as "no", this is a finding.

Fix

Configure the SSH daemon to prevent remote hosts from connecting to the proxy display.

Add the following line to "/etc/ssh/sshd_config", or uncomment the line and set the value to "yes":

X11UseLocalhost yes

Alternatively, add the setting to an include file if the line "Include /etc/ssh/sshd_config.d/*.conf" is found at the top of the "/etc/ssh/sshd_config" file:

$ echo 'X11UseLocalhost yes' > /etc/ssh/sshd_config.d/40-x11local.conf

Restart the SSH daemon for the settings to take effect:

$ systemctl restart sshd.service