Verify Permissions on SSH Server Public *.pub Key Files

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Description

To properly set the permissions of /etc/ssh/*.pub, run the command:

$ sudo chmod 0644 /etc/ssh/*.pub

Rationale

If a public host key file is modified by an unauthorized user, the SSH service may be compromised.

Remediation

Shell script

The following script can be run on the host to remediate the issue.

#!/bin/bash

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

find -H /etc/ssh/ -maxdepth 1 -perm /u+xs,g+xws,o+xwt  -type f -regextype posix-extended -regex '^.*\.pub$' -exec chmod u-xs,g-xws,o-xwt {} \;

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

Ansible playbook

The following playbook can be run with Ansible to remediate the issue.

- name: Find /etc/ssh/ file(s)
  command: find -H /etc/ssh/ -maxdepth 1 -perm /u+xs,g+xws,o+xwt  -type f -regextype
    posix-extended -regex "^.*\.pub$"
  register: files_found
  changed_when: false
  failed_when: false
  check_mode: false
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-171-3.1.13
  - NIST-800-171-3.13.10
  - NIST-800-53-AC-17(a)
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - PCI-DSS-Req-2.2.4
  - PCI-DSSv4-2.2.6
  - configure_strategy
  - file_permissions_sshd_pub_key
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed

- name: Set permissions for /etc/ssh/ file(s)
  file:
    path: '{{ item }}'
    mode: u-xs,g-xws,o-xwt
    state: file
  with_items:
  - '{{ files_found.stdout_lines }}'
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - NIST-800-171-3.1.13
  - NIST-800-171-3.13.10
  - NIST-800-53-AC-17(a)
  - NIST-800-53-AC-6(1)
  - NIST-800-53-CM-6(a)
  - PCI-DSS-Req-2.2.4
  - PCI-DSSv4-2.2.6
  - configure_strategy
  - file_permissions_sshd_pub_key
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed