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

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-258011	RHEL-09-255175	SV-258011r943050_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
Red Hat Enterprise Linux 9 Security Technical Implementation Guide	2023-12-01

Details

Check Text ( C-61752r943049_chk )
Verify the SSH daemon prevents remote hosts from connecting to the proxy display with the following command: $ sudo grep -ir x11uselocal /etc/ssh/sshd_config /etc/ssh/sshd_config.d/* X11UseLocalhost yes If the "X11UseLocalhost" keyword is set to "no", is missing, or is commented out, this is a finding.

Fix Text (F-61676r926019_fix)
Configure the SSH daemon to prevent remote hosts from connecting to the proxy display. Add the following line in "/etc/ssh/sshd_config", 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