SV-258011r1045079: RHEL 9 SSH daemon must prevent remote hosts from connecting to the proxy display. | Red Hat Enterprise Linux 9 STIG SCAP Benchmark

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Assessment Title

Rule Text

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

Description

Discussion Text

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.

How this relates to the STIG checklist: STIG vs SCAP benchmark.

IA Controls

NIST SP 800-53

Not mappedCCI-000366

References

DPMS Target Red Hat Enterprise Linux 9 DISA DPMS Target Red Hat Enterprise Linux 9 5551

SCC Automation Reference

The reconstructed evaluation script behind this rule's OVAL check(s) — every module a compound check combines, with this rule's own concrete values and DISA's SCC tool's real per-module mechanism curated from static teardown of SCC releases.

#!/bin/bash
# Reconstructed from oval:mil.disa.stig.rhel9os:def:258011
# RHEL 9 SSH daemon must prevent remote hosts from connecting to the proxy display.
# Per-module read mechanism curated from static teardown of DISA's SCC tool
# (SCAP Compliance Checker) -- see the SCC Automation Reference above.

# sshd_config X11UseLocalhost is set to 'yes'.
# textfilecontent54_object (textfilecontent54_test)
test_1() {
  ( (sed -En 's/^\s*(?i)X11useLocalhost(?-i)[ \t]+([\w\"]+)[\s]*(?:|(?:#.*))?$/\1/p' '/etc/ssh/sshd_config') | grep -Pq '^(yes|"yes")$' ) || ( (grep -rhP --include='*.conf' '^\s*(?i)X11useLocalhost(?-i)[ \t]+([\w\"]+)[\s]*(?:|(?:#.*))?$' '/etc/ssh/sshd_config.d' | sed -En 's/^\s*(?i)X11useLocalhost(?-i)[ \t]+([\w\"]+)[\s]*(?:|(?:#.*))?$/\1/p') | grep -Pq '^(yes|"yes")$' )
}

if test_1; then
  echo "PASS"
else
  echo "FAIL"
fi

Assessment Script Powered by AISandbox-executed

A standalone Shell script to assess this control automatically. Review before running it in your environment.

Syntax-checked and actually executed in an isolated sandbox (not the real system this checks) — confirmed to run without errors.

#!/bin/sh
# OVAL Definition: oval:mil.disa.stig.rhel9os:def:258011
if [ ! -f "/etc/ssh/sshd_config" ] && [ ! -d "/etc/ssh/sshd_config.d" ]; then echo "PASS"; exit 0; fi
if [ -f "/etc/ssh/sshd_config" ]; then if grep -iqE "^(yes|\"yes\")$" /etc/ssh/sshd_config; then echo "PASS"; exit 0; fi; fi
for file in /etc/ssh/sshd_config.d/*.conf; do if [ -f "$file" ]; then if grep -iqE "^(yes|\"yes\")$" "$file"; then echo "PASS"; exit 0; fi; fi; done
echo "FAIL"; exit 1

Remediation

Configure the SSH daemon to prevent remote hosts from connecting to the proxy display.

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 "yes":

X11UseLocalhost yes

The SSH service must be restarted for changes to take effect:

$ sudo systemctl restart sshd.service