Enable SSH Warning Banner

An XCCDF Rule

Description

To enable the warning banner and ensure it is consistent across the system, add or correct the following line in /etc/ssh/sshd_config:

Banner /etc/issue.net

Another section contains information on how to create an appropriate system-wide warning banner.

Rationale

The warning message reinforces policy awareness during the logon process and facilitates possible legal action against attackers. Alternatively, systems whose ownership should not be obvious should ensure usage of a banner that does not provide easy attribution.

ID: xccdf_org.ssgproject.content_rule_sshd_enable_warning_banner_net
Severity: Medium
References: 5.5.6

DSS05.04 DSS05.10 DSS06.10

3.1.9

CCI-000048 CCI-000050 CCI-001384 CCI-001385 CCI-001386 CCI-001387 CCI-001388

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)

4.3.3.6.1 4.3.3.6.2 4.3.3.6.3 4.3.3.6.4 4.3.3.6.5 4.3.3.6.6 4.3.3.6.7 4.3.3.6.8 4.3.3.6.9

SR 1.1 SR 1.10 SR 1.2 SR 1.5 SR 1.7 SR 1.8 SR 1.9

A.18.1.4 A.9.2.1 A.9.2.4 A.9.3.1 A.9.4.2 A.9.4.3

AC-17(a) AC-8(a) AC-8(c) CM-6(a)

PR.AC-7

FTA_TAB.1

SRG-OS-000023-GPOS-00006 SRG-OS-000228-GPOS-00088
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:
  - CJIS-5.5.6
  - NIST-800-171-3.1.9
  - NIST-800-53-AC-17(a)
  - NIST-800-53-AC-8(a)
  - NIST-800-53-AC-8(c)
  - NIST-800-53-CM-6(a)
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_enable_warning_banner_net

- name: Comment conf from included files
  replace:
    path: '{{ item }}'
    regexp: ^(\s*Banner.*)$
    replace: '# \1'
  loop: '{{ sshd_config_included_files.stdout_lines }}'
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CJIS-5.5.6
  - NIST-800-171-3.1.9
  - NIST-800-53-AC-17(a)
  - NIST-800-53-AC-8(a)
  - NIST-800-53-AC-8(c)
  - NIST-800-53-CM-6(a)
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_enable_warning_banner_net

- name: Enable SSH Warning Banner
  block:

  - name: Check for duplicate values
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)^\s*Banner\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*Banner\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*Banner\s+
      line: Banner /etc/issue.net
      state: present
      insertbefore: BOF
      validate: /usr/sbin/sshd -t -f %s
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - CJIS-5.5.6
  - NIST-800-171-3.1.9
  - NIST-800-53-AC-17(a)
  - NIST-800-53-AC-8(a)
  - NIST-800-53-AC-8(c)
  - NIST-800-53-CM-6(a)
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_enable_warning_banner_net

# 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*Banner/Id" "$included_file"
done

if [ -e "/etc/ssh/sshd_config" ] ; then
    
    LC_ALL=C sed -i "/^\s*Banner\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' "Banner /etc/issue.net" > "/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

Enable SSH Warning Banner

Description

Rationale

Remediation - Ansible

Remediation - Shell Script