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.d/00-complianceascode-hardening.conf:

Banner /etc/issue

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
Severity: Medium
References: 1 12 15 16

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

Req-2.2.4

SRG-OS-000023-GPOS-00006 SRG-OS-000228-GPOS-00088
Updated: about 1 year ago

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

mkdir -p /etc/ssh/sshd_config.d
touch /etc/ssh/sshd_config.d/00-complianceascode-hardening.conf
LC_ALL=C sed -i "/^\s*Banner\s\+/Id" "/etc/ssh/sshd_config"
LC_ALL=C sed -i "/^\s*Banner\s\+/Id" "/etc/ssh/sshd_config.d"/*.conf
if [ -e "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf" ] ; then
    
    LC_ALL=C sed -i "/^\s*Banner\s\+/Id" "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf"
else
    touch "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf"
fi
# make sure file has newline at the end
sed -i -e '$a\' "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf"

cp "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf" "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf.bak"
# Insert at the beginning of the file
printf '%s\n' "Banner /etc/issue" > "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf"
cat "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf.bak" >> "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf"
# Clean up after ourselves.
rm "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf.bak"

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

- name: Enable SSH Warning Banner
  block:

  - name: Deduplicate values from /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config      create: false
      regexp: (?i)^\s*{{ "Banner"| regex_escape }}\s+
      state: absent

  - name: Check if /etc/ssh/sshd_config.d exists
    stat:
      path: /etc/ssh/sshd_config.d
    register: _etc_ssh_sshd_config_d_exists

  - name: Check if the parameter Banner is present in /etc/ssh/sshd_config.d
    find:
      paths: /etc/ssh/sshd_config.d
      recurse: 'yes'
      follow: 'no'
      contains: (?i)^\s*{{ "Banner"| regex_escape }}\s+
    register: _etc_ssh_sshd_config_d_has_parameter
    when: _etc_ssh_sshd_config_d_exists.stat.isdir is defined and _etc_ssh_sshd_config_d_exists.stat.isdir

  - name: Remove parameter from files in /etc/ssh/sshd_config.d
    lineinfile:
      path: '{{ item.path }}'
      create: false
      regexp: (?i)^\s*{{ "Banner"| regex_escape }}\s+
      state: absent
    with_items: '{{ _etc_ssh_sshd_config_d_has_parameter.files }}'
    when: _etc_ssh_sshd_config_d_has_parameter.matched

  - name: Insert correct line to /etc/ssh/sshd_config.d/00-complianceascode-hardening.conf
    lineinfile:
      path: /etc/ssh/sshd_config.d/00-complianceascode-hardening.conf
      create: true
      regexp: (?i)^\s*{{ "Banner"| regex_escape }}\s+
      line: Banner /etc/issue
      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)
  - PCI-DSS-Req-2.2.4
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_enable_warning_banner

Enable SSH Warning Banner

Description

Rationale

Remediation - Shell Script

Remediation - Ansible