Title

RHEL 9 SSH daemon must disable remote X connections for interactive users. (Cat II impact)

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.

Check Content

Verify the SSH daemon does not allow X11Forwarding 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*x11forwarding' X11forwarding no If the value is returned as "yes", the returned line is commented out, or no output is returned, and X11 forwarding is not documented with the information system security officer (ISSO) as an operational requirement, this is a finding.

Fix Text

Configure the SSH daemon to not allow X11 forwarding. Add the following line in "/etc/ssh/sshd_config", or uncomment the line and set the value to "no": X11forwarding no The SSH service must be restarted for changes to take effect: $ sudo systemctl restart sshd.service

Additional Identifiers

Rule ID: SV-258007r952210_rule

Vulnerability ID: V-258007

Group Title: SRG-OS-000480-GPOS-00227

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