Enable Public Key Authentication

An XCCDF Rule

Description

Enable SSH login with public keys.
The default SSH configuration enables authentication based on public keys. The appropriate configuration is used if no value is set for PubkeyAuthentication.
To explicitly enable Public Key Authentication, add or correct the following /etc/ssh/sshd_config:

PubkeyAuthentication yes

Rationale

Without the use of multifactor authentication, the ease of access to privileged functions is greatly increased. Multifactor authentication requires using two or more factors to achieve authentication. A privileged account is defined as an information system account with authorizations of a privileged user. The DoD CAC with DoD-approved PKI is an example of multifactor authentication.

ID: xccdf_org.ssgproject.content_rule_sshd_enable_pubkey_auth
Severity: Medium
References: CCI-000765 CCI-000766 CCI-000767 CCI-000768

SRG-OS-000105-GPOS-00052 SRG-OS-000106-GPOS-00053 SRG-OS-000107-GPOS-00054 SRG-OS-000108-GPOS-00055
Updated: about 1 year ago

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

if [ -e "/etc/ssh/sshd_config" ] ; then
    
    LC_ALL=C sed -i "/^\s*PubkeyAuthentication\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' "PubkeyAuthentication yes" > "/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

- name: Enable Public Key Authentication
  block:

  - name: Check for duplicate values
    lineinfile:
      path: /etc/ssh/sshd_config      create: true
      regexp: (?i)^\s*PubkeyAuthentication\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*PubkeyAuthentication\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*PubkeyAuthentication\s+
      line: PubkeyAuthentication yes
      state: present
      insertbefore: BOF
      validate: /usr/sbin/sshd -t -f %s
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_enable_pubkey_auth

Enable Public Key Authentication

Description

Rationale

Remediation - Shell Script

Remediation - Ansible