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V-258007

CAT II (Medium)

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

Rule ID

SV-258007r1045073_rule

STIG

Red Hat Enterprise Linux 9 Security Technical Implementation Guide

Version

V2R8

CCIs

CCI-000366

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 to "/etc/ssh/sshd_config" or to a file in "/etc/ssh/sshd_config.d", 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