SV-258004r1045067: RHEL 9 SSH daemon must not allow Kerberos authentication. | Red Hat Enterprise Linux 9 STIG SCAP Benchmark

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Assessment Title

Rule Text

RHEL 9 SSH daemon must not allow Kerberos authentication.

Description

Discussion Text

Kerberos authentication for SSH is often implemented using Generic Security Service Application Program Interface (GSSAPI). If Kerberos is enabled through SSH, the SSH daemon provides a means of access to the system's Kerberos implementation. Vulnerabilities in the system's Kerberos implementations may be subject to exploitation. Satisfies: SRG-OS-000364-GPOS-00151, SRG-OS-000480-GPOS-00227

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

IA Controls

NIST SP 800-53

Not mappedCCI-001813

References

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

Checks

DISA STIG (Manual Reference)

Verify the SSH daemon does not allow Kerberos authentication 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*kerberosauthentication'

KerberosAuthentication no

If the value is returned as "yes", the returned line is commented out, no output is returned, and the use of Kerberos authentication has not been documented with the information system security officer (ISSO), this is a finding.

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:258004
# RHEL 9 SSH daemon must not allow Kerberos authentication.
# Per-module read mechanism curated from static teardown of DISA's SCC tool
# (SCAP Compliance Checker) -- see the SCC Automation Reference above.

# /etc/ssh/sshd_config:KerberosAuthentication == no
# textfilecontent54_object (textfilecontent54_test)
test_1() {
  (sed -En 's/^\s*(?i)KerberosAuthentication(?-i)\s+("?\w+"?)\s*(?:|(?:#.*))?$/\1/p' '/etc/ssh/sshd_config') | grep -Pq '^(no|"no")$'
}
# /etc/ssh/sshd_config does not include sshd_config.d
# textfilecontent54_object (textfilecontent54_test)
test_2() {
  (sed -En 's/^\s*include\s+(.*)\s*$/\1/p' '/etc/ssh/sshd_config') | grep -qxF '/etc/ssh/sshd_config.d/*.conf'
}
# /etc/ssh/sshd_config includes sshd_config.d
# textfilecontent54_object (textfilecontent54_test)
test_3() {
  (sed -En 's/^\s*include\s+(.*)\s*$/\1/p' '/etc/ssh/sshd_config') | grep -qxF '/etc/ssh/sshd_config.d/*.conf'
}
# /etc/ssh/sshd_config + sshd_config.d:KerberosAuthentication == no
# textfilecontent54_object (textfilecontent54_test)
test_4() {
  ( (sed -En 's/^\s*(?i)KerberosAuthentication(?-i)\s+("?\w+"?)\s*(?:|(?:#.*))?$/\1/p' '/etc/ssh/sshd_config') | grep -Pq '^(no|"no")$' ) || ( (grep -rhP --include='*.conf' '^\s*(?i)KerberosAuthentication(?-i)\s+("?\w+"?)\s*(?:|(?:#.*))?$' '/etc/ssh/sshd_config.d' | sed -En 's/^\s*(?i)KerberosAuthentication(?-i)\s+("?\w+"?)\s*(?:|(?:#.*))?$/\1/p') | grep -Pq '^(no|"no")$' )
}

if ( ( test_1 && test_2 ) || ( test_3 && test_4 ) ); 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
# DISA STIG RHEL 9 SSH daemon must not allow Kerberos authentication check

if [ ! -f "/etc/ssh/sshd_config" ]; then
  echo "PASS"
  exit 0
fi

if grep -qE '^\\s*(?i)KerberosAuthentication(?-i)\\s+(\"?no\"?\\?)\\s*' /etc/ssh/sshd_config; then
  echo "PASS"
  exit 0
fi

if [ ! -d "/etc/ssh/sshd_config.d" ]; then
  echo "PASS"
  exit 0
fi

for file in /etc/ssh/sshd_config.d/*.conf; do
  if grep -qE '^\\s*(?i)KerberosAuthentication(?-i)\\s+(\"?no\"?\\?)\\s*' "$file"; then
    echo "PASS"
    exit 0
  fi
done

echo "FAIL"
exit 1

Remediation

Configure the SSH daemon to not allow Kerberos authentication.

Add the following line in "/etc/ssh/sshd_config" or to a file in "/etc/ssh/sshd_config.d", or uncomment the line and set the value to "no":

KerberosAuthentication no

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

$ sudo systemctl restart sshd.service