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Limit the MACs to strong hash algorithms.
The following line in /etc/ssh/sshd_config demonstrates use
of those MACs:
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256
Rationale
MD5 and 96-bit MAC algorithms are considered weak and have been shown to increase
exploitability in SSH downgrade attacks. Weak algorithms continue to have a great deal of
attention as a weak spot that can be exploited with expanded computing power. An
attacker that breaks the algorithm could take advantage of a MiTM position to decrypt the
SSH tunnel and capture credentials and information
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
sshd_strong_macs='hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256'
# 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=$(sed 's/[\^=\$,;+]*//g' <<< "^MACs")
# shellcheck disable=SC2059
printf -v formatted_output "%s %s" "$stripped_key" "$sshd_strong_macs"
# If the key exists, change it. Otherwise, add it to the config_file.
# We search for the key string followed by a word boundary (matched by \>),
# so if we search for 'setting', 'setting2' won't match.
if LC_ALL=C grep -q -m 1 -i -e "^MACs\\>" "/etc/ssh/sshd_config"; then
escaped_formatted_output=$(sed -e 's|/|\\/|g' <<< "$formatted_output")
LC_ALL=C sed -i --follow-symlinks "s/^MACs\\>.*/$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
Ansible playbook
The following playbook can be run with Ansible to remediate the issue.
- name: XCCDF Value sshd_strong_macs # promote to variable
set_fact:
sshd_strong_macs: !!str hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256
tags:
- always
- name: Use Only Strong MACs
block:
- name: Check for duplicate values
lineinfile:
path: /etc/ssh/sshd_config
create: true
regexp: (?i)^\s*MACs\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*MACs\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*MACs\s+
line: MACs {{ sshd_strong_macs }}
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_use_strong_macs
