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Limit the ciphers to those algorithms which are FIPS-approved.
Counter (CTR) mode is also preferred over cipher-block chaining (CBC) mode.
The following line in /etc/ssh/sshd_config
demonstrates use of FIPS-approved ciphers:
Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc
The man page sshd_config(5)
contains a list of supported ciphers.
The rule is parametrized to use the following ciphers: aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se
.
Unapproved mechanisms that are used for authentication to the cryptographic module are not verified and therefore cannot be relied upon to provide confidentiality or integrity, and system data may be compromised.
Operating systems utilizing encryption are required to use FIPS-compliant mechanisms for authenticating to cryptographic modules.
FIPS 140-2 is the current standard for validating that mechanisms used to access cryptographic modules utilize authentication that meets industry and government requirements. For government systems, this allows Security Levels 1, 2, 3, or 4 for use on Ubuntu 20.04.
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
sshd_approved_ciphers='aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se'
# Strip any search characters in the key arg so that the key can be replaced without
# adding any search characters to the config file.
stripped_key=$(sed 's/[\^=\$,;+]*//g' <<< "^Ciphers")
# shellcheck disable=SC2059
printf -v formatted_output "%s %s" "$stripped_key" "$sshd_approved_ciphers"
# If the key exists, change it. Otherwise, add it to the config_file.
# We search for the key string followed by a word boundary (matched by \>),
# so if we search for 'setting', 'setting2' won't match.
if LC_ALL=C grep -q -m 1 -i -e "^Ciphers\\>" "/etc/ssh/sshd_config"; then
escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
LC_ALL=C sed -i --follow-symlinks "s/^Ciphers\\>.*/$escaped_formatted_output/gi" "/etc/ssh/sshd_config"
else
if [[ -s "/etc/ssh/sshd_config" ]] && [[ -n "$(tail -c 1 -- "/etc/ssh/sshd_config" || true)" ]]; then
LC_ALL=C sed -i --follow-symlinks '$a'\\ "/etc/ssh/sshd_config"
fi
printf '%s\n' "$formatted_output" >> "/etc/ssh/sshd_config"
fi
else
>&2 echo 'Remediation is not applicable, nothing was done'
fi
The following playbook can be run with Ansible to remediate the issue.
- name: XCCDF Value sshd_approved_ciphers # promote to variable
set_fact:
sshd_approved_ciphers: !!str aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se
tags:
- always
- name: Use Only FIPS 140-2 Validated Ciphers
block:
- name: Check for duplicate values
lineinfile:
path: /etc/ssh/sshd_config
create: true
regexp: (?i)^\s*Ciphers\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*Ciphers\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*Ciphers\s+
line: Ciphers {{ sshd_approved_ciphers }}
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-800-171-3.1.13
- NIST-800-171-3.13.11
- NIST-800-171-3.13.8
- NIST-800-53-AC-17(2)
- NIST-800-53-AC-17(a)
- NIST-800-53-CM-6(a)
- NIST-800-53-IA-5(1)(c)
- NIST-800-53-MA-4(6)
- NIST-800-53-SC-12(2)
- NIST-800-53-SC-12(3)
- NIST-800-53-SC-13
- PCI-DSSv4-2.2.7
- low_complexity
- low_disruption
- medium_severity
- no_reboot_needed
- restrict_strategy
- sshd_use_approved_ciphers
The system needs to be rebooted for these changes to take effect.