← Back to Canonical Ubuntu 22.04 LTS Security Technical Implementation Guide

V-260530

CAT II (Medium)

Ubuntu 22.04 LTS SSH daemon must prevent remote hosts from connecting to the proxy display.

Rule ID

SV-260530r991589_rule

STIG

Canonical Ubuntu 22.04 LTS Security Technical Implementation Guide

Version

V2R8

CCIs

CCI-000366

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 Content

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

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