AlmaLinux OS 9 SSH daemon must disable remote X connections for interactive users.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-269270	SRG-OS-000480-GPOS-00227	ALMA-09-021470	SV-269270r1050152_rule	2025-02-20	1

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.

ℹ️ Check
Verify the SSH daemon does not allow X11Forwarding with the following command: $ sshd -T \| grep x11forwarding x11forwarding no If the value is returned as "yes" and X11 forwarding is not documented with the information system security officer (ISSO) as an operational requirement, this is a finding.

✔️ Fix
Configure the SSH daemon to not allow X11 forwarding. Add the following line to "/etc/ssh/sshd_config", or uncomment the line and set the value to "no": X11forwarding no Alternatively, add the setting to an include file if the line "Include /etc/ssh/sshd_config.d/*.conf" is found at the top of the "/etc/ssh/sshd_config" file: $ echo 'X11forwarding no' > /etc/ssh/sshd_config.d/40-x11forwarding.conf Restart the SSH daemon for the settings to take effect: $ systemctl restart sshd.service

✔️ Fix

Configure the SSH daemon to not allow X11 forwarding. Add the following line to "/etc/ssh/sshd_config", or uncomment the line and set the value to "no": X11forwarding no Alternatively, add the setting to an include file if the line "Include /etc/ssh/sshd_config.d/*.conf" is found at the top of the "/etc/ssh/sshd_config" file: $ echo 'X11forwarding no' > /etc/ssh/sshd_config.d/40-x11forwarding.conf Restart the SSH daemon for the settings to take effect: $ systemctl restart sshd.service