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V-238220

CAT II (Medium)

The Ubuntu operating system SSH daemon must prevent remote hosts from connecting to the proxy display.

Rule ID

SV-238220r991589_rule

STIG

Canonical Ubuntu 20.04 LTS Security Technical Implementation Guide

Version

V2R4

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 daemon prevents remote hosts from connecting to the proxy display.

Check the SSH X11UseLocalhost setting with the following command:

$ sudo /usr/sbin/sshd -dd 2>&1 | awk '/filename/ {print $4}' | tr -d '\r' | tr '\n' ' ' | xargs sudo grep -iH '^\s*x11uselocalhost'

X11UseLocalhost yes

If the "X11UseLocalhost" keyword is set to "no", is missing, or is commented out, this is a finding.

If conflicting results are returned, this is a finding.

Fix Text

Configure the SSH daemon to prevent remote hosts from connecting to the proxy display. 
 
Edit the "/etc/ssh/sshd_config" file to uncomment or add the line for the "X11UseLocalhost" keyword and set its value to "yes" (this file may be named differently or be in a different location if using a version of SSH that is provided by a third-party vendor): 
 
X11UseLocalhost yes 
 
Restart the SSH daemon for the changes to take effect: 
 
$ sudo systemctl restart sshd.service