Ensure SSH X11 forwarding is disabled

MediumLevel 1Automatedcos

Description

The X11Forwarding parameter provides the ability to tunnel X11 traffic through the connection to enable remote graphic connections. Disable X11 forwarding unless there is an operational requirement to use X11 applications directly. There is a small risk that the remote X11 servers of users who are logged in via SSH with X11 forwarding could be compromised by other users on the X11 server. Note that even if X11 forwarding is disabled, users can always install their own forwarders.

How it is checked

Audit method: Shell command

shell

alt0() { command -v grep >/dev/null 2>&1 || { echo "step1: grep not installed"; return 1; }; command -v sshd >/dev/null 2>&1 || { echo "step1: sshd not installed"; return 1; }; o=$(bash -c 'sshd -T | grep x11forwarding' 2>&1); printf '%s\n' "$o" | tr -s ' \t' '  ' | sed 's/^ //;s/ $//' | grep -Fxq -- 'X11Forwarding no' || { echo step1: missing 'X11Forwarding no'; return 1; }; }; if alt0; then echo compliant; fi

Expected output: compliant

How it is fixed

Fix method: Guidance or infrastructure patch

Edit the /etc/ssh/sshd_config file to set the parameter as follows: X11Forwarding no

Properties

Operating systems: Any
Tags: container-optimized-oscosssh
Reference: CIS Google Container-Optimized OS Benchmark v1.2.0 - 5.1.6 Ensure SSH X11 forwarding is disabled

Check this host

Run this on a machine you are allowed to audit. The scan is read-only.

shell

tmaegis scan --target cos