Disable Kerberos Authentication

An XCCDF Rule

Description

Unless needed, SSH should not permit extraneous or unnecessary authentication mechanisms like Kerberos.
The default SSH configuration disallows authentication validation through Kerberos. The appropriate configuration is used if no value is set for KerberosAuthentication.
To explicitly disable Kerberos authentication, add or correct the following line in /etc/ssh/sshd_config:

KerberosAuthentication no

Rationale

Kerberos authentication for SSH is often implemented using GSSAPI. If Kerberos is enabled through SSH, the SSH daemon provides a means of access to the system's Kerberos implementation. Configuring these settings for the SSH daemon provides additional assurance that remote logon via SSH will not use unused methods of authentication, even in the event of misconfiguration elsewhere.

ID: xccdf_org.ssgproject.content_rule_sshd_disable_kerb_auth
Severity: Medium
References: 11 3 9

BAI10.01 BAI10.02 BAI10.03 BAI10.05

3.1.12

CCI-000318 CCI-000366 CCI-000368 CCI-001812 CCI-001813 CCI-001814

164.308(a)(4)(i) 164.308(b)(1) 164.308(b)(3) 164.310(b) 164.312(e)(1) 164.312(e)(2)(ii)

4.3.4.3.2 4.3.4.3.3

SR 7.6

0421 0422 0431 0974 1173 1401 1504 1505 1546 1557 1558 1559 1560 1561

A.12.1.2 A.12.5.1 A.12.6.2 A.14.2.2 A.14.2.3 A.14.2.4

AC-17(a) CM-6(a) CM-7(a) CM-7(b)

PR.IP-1

FCS_SSH_EXT.1.2 FTP_ITC_EXT.1

SRG-OS-000364-GPOS-00151 SRG-OS-000480-GPOS-00227
Updated: about 1 year ago

- name: Find sshd_config included files
  shell: |-
    included_files=$(grep -oP "^\s*(?i)include.*" /etc/ssh/sshd_config | sed -e 's/\s*Include\s*//i' | sed -e 's|^[^/]|/etc/ssh/&|')
    [[ -n $included_files ]] && ls $included_files || true
  register: sshd_config_included_files
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]  tags:
  - NIST-800-171-3.1.12
  - NIST-800-53-AC-17(a)
  - NIST-800-53-CM-6(a)
  - NIST-800-53-CM-7(a)
  - NIST-800-53-CM-7(b)
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_disable_kerb_auth

- name: Comment conf from included files
  replace:
    path: '{{ item }}'
    regexp: ^(\s*KerberosAuthentication.*)$
    replace: '# \1'
  loop: '{{ sshd_config_included_files.stdout_lines }}'
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-171-3.1.12
  - NIST-800-53-AC-17(a)
  - NIST-800-53-CM-6(a)
  - NIST-800-53-CM-7(a)
  - NIST-800-53-CM-7(b)
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_disable_kerb_auth

- name: Disable Kerberos Authentication
  block:

  - name: Check for duplicate values
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*KerberosAuthentication\s+
      state: absent
    check_mode: true
    changed_when: false
    register: dupes

  - name: Deduplicate values from /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*KerberosAuthentication\s+
      state: absent
    when: dupes.found is defined and dupes.found > 1

  - name: Insert correct line to /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*KerberosAuthentication\s+
      line: KerberosAuthentication no
      state: present
      insertbefore: BOF
      validate: /usr/sbin/sshd -t -f %s
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-171-3.1.12
  - NIST-800-53-AC-17(a)
  - NIST-800-53-CM-6(a)
  - NIST-800-53-CM-7(a)
  - NIST-800-53-CM-7(b)
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_disable_kerb_auth

# Remediation is applicable only in certain platforms
if [ ! -f /.dockerenv ] && [ ! -f /run/.containerenv ]; then

# Find the include keyword, extract from the line the glob expression representing included files.
# And if it is a relative path prepend '/etc/ssh/'
included_files=$(grep -oP "^\s*(?i)include.*" /etc/ssh/sshd_config | sed -e 's/\s*include\s*//I' | sed -e 's|^[^/]|/etc/ssh/&|')for included_file in ${included_files} ; do
    
    LC_ALL=C sed -i "/^\s*KerberosAuthentication/Id" "$included_file"
done

if [ -e "/etc/ssh/sshd_config" ] ; then
    
    LC_ALL=C sed -i "/^\s*KerberosAuthentication\s\+/Id" "/etc/ssh/sshd_config"
else
    touch "/etc/ssh/sshd_config"
fi
# make sure file has newline at the end
sed -i -e '$a\' "/etc/ssh/sshd_config"

cp "/etc/ssh/sshd_config" "/etc/ssh/sshd_config.bak"
# Insert at the beginning of the file
printf '%s\n' "KerberosAuthentication no" > "/etc/ssh/sshd_config"
cat "/etc/ssh/sshd_config.bak" >> "/etc/ssh/sshd_config"
# Clean up after ourselves.
rm "/etc/ssh/sshd_config.bak"

else
    >&2 echo 'Remediation is not applicable, nothing was done'
fi

Disable Kerberos Authentication

Description

Rationale

Remediation - Ansible

Remediation - Shell Script