Set SSH authentication attempt limit
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Description
The MaxAuthTries
parameter specifies the maximum number of authentication attempts
permitted per connection. Once the number of failures reaches half this value, additional failures are logged.
to set MaxAUthTries edit /etc/ssh/sshd_config
as follows:
Rationale
Setting the MaxAuthTries parameter to a low number will minimize the risk of successful
brute force attacks to the SSH server.
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 dpkg-query --show --showformat='${db:Status-Status}
' 'kernel' 2>/dev/null | grep -q installed; then
sshd_max_auth_tries_value='4'
mkdir -p /etc/ssh/sshd_config.d
touch /etc/ssh/sshd_config.d/00-complianceascode-hardening.conf
LC_ALL=C sed -i "/^\s*MaxAuthTries\s\+/Id" "/etc/ssh/sshd_config"
LC_ALL=C sed -i "/^\s*MaxAuthTries\s\+/Id" "/etc/ssh/sshd_config.d"/*.conf
if [ -e "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf" ] ; then
LC_ALL=C sed -i "/^\s*MaxAuthTries\s\+/Id" "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf"
else
touch "/etc/ssh/sshd_config.d/00-complianceascode-hardening.conf"
fi
# make sure file has newline at the end
sed -i -e '$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' "MaxAuthTries $sshd_max_auth_tries_value" > "/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
Ansible playbook
The following playbook can be run with Ansible to remediate the issue.
- name: Gather the package facts
package_facts:
manager: auto
tags:
- PCI-DSSv4-2.2
- PCI-DSSv4-2.2.6
- low_complexity
- low_disruption
- medium_severity
- no_reboot_needed
- restrict_strategy
- sshd_set_max_auth_tries
- name: XCCDF Value sshd_max_auth_tries_value # promote to variable
set_fact:
sshd_max_auth_tries_value: !!str 4
tags:
- always
- name: Set SSH authentication attempt limit
block:
- name: Deduplicate values from /etc/ssh/sshd_config
lineinfile:
path: /etc/ssh/sshd_config
create: false
regexp: (?i)(?i)^\s*{{ "MaxAuthTries"| 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 MaxAuthTries is present in /etc/ssh/sshd_config.d
find:
paths: /etc/ssh/sshd_config.d
recurse: 'yes'
follow: 'no'
contains: (?i)^\s*{{ "MaxAuthTries"| 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)(?i)^\s*{{ "MaxAuthTries"| 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)(?i)^\s*{{ "MaxAuthTries"| regex_escape }}\s+
line: MaxAuthTries {{ sshd_max_auth_tries_value }}
state: present
insertbefore: BOF
validate: /usr/sbin/sshd -t -f %s
when: '"kernel" in ansible_facts.packages'
tags:
- PCI-DSSv4-2.2
- PCI-DSSv4-2.2.6
- low_complexity
- low_disruption
- medium_severity
- no_reboot_needed
- restrict_strategy
- sshd_set_max_auth_tries