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UBTU-22-255045 Ubuntu 22.04 LTS SSH daemon must prevent remote hosts from connecting to the proxy display.

DISA STIG library V2R10
All changes in this version
first version loaded CAT II

Maps to CM-6 via CCI-000366

Title

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

Control

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.

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.

Related controls

CCI-000366

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 SSH for the changes to take effect: $ sudo systemctl restart ssh