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RHEL-09-255175 RHEL 9 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

RHEL 9 SSH daemon must prevent remote hosts from connecting to the proxy display.

Control

Verify the SSH daemon prevents remote hosts from connecting to the proxy display 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.

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 daemon to prevent remote hosts from connecting to the proxy display. Add the following line to "/etc/ssh/sshd_config" or to a file in "/etc/ssh/sshd_config.d", or uncomment the line and set the value to "yes": X11UseLocalhost yes The SSH service must be restarted for changes to take effect: $ sudo systemctl restart sshd.service