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		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	2024-05-30

Details

Check Text ( C-64259r953401_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.

Check Text ( C-64259r953401_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