Ubuntu 22.04 LTS SSH daemon must prevent remote hosts from connecting to the proxy display.
Overview
| Finding ID | Version | Rule ID | IA Controls | Severity |
| V-260530 | UBTU-22-255045 | SV-260530r991589_rule | CCI-000366 | medium |
| Description | ||||
| 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. | ||||
| STIG | Date | |||
| Canonical Ubuntu 22.04 LTS Security Technical Implementation Guide | 2025-05-16 | |||
Details
Check Text (C-260530r991589_chk)
Verify the SSH server prevents remote hosts from connecting to the proxy display by using the following command:
$ sudo /usr/sbin/sshd -dd 2>&1 | awk '/filename/ {print $4}' | tr -d '\r' | tr '\n' ' ' | xargs sudo grep -iH 'x11uselocalhost'
/etc/ssh/sshd_config:X11UseLocalhost yes
If "X11UseLocalhost" is set to "no", is commented out, is missing, or conflicting results are returned, this is a finding.
Fix Text (F-64167r953402_fix)
Configure the SSH server to prevent remote hosts from connecting to the proxy display.
Add or modify the following line in the "/etc/ssh/sshd_config" file:
X11UseLocalhost yes
Restart the SSH daemon for the changes to take effect:
$ sudo systemctl restart sshd.service