Force frequent session key renegotiation

An XCCDF Rule

Description

The RekeyLimit parameter specifies how often the session key of the is renegotiated, both in terms of amount of data that may be transmitted and the time elapsed.
To decrease the default limits, add or correct the following line in /etc/ssh/sshd_config.d/00-complianceascode-hardening.conf:

RekeyLimit

Rationale

By decreasing the limit based on the amount of data and enabling time-based limit, effects of potential attacks against encryption keys are limited.

ID: xccdf_org.ssgproject.content_rule_sshd_rekey_limit
Severity: Medium
References: CCI-000068

FCS_SSH_EXT.1.8

SRG-OS-000033-GPOS-00014 SRG-OS-000480-GPOS-00227
Updated: about 1 year ago

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

var_rekey_limit_size='<xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_rekey_limit_size" use="legacy"/>'
var_rekey_limit_time='<xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_rekey_limit_time" use="legacy"/>'


mkdir -p /etc/ssh/sshd_config.d
touch /etc/ssh/sshd_config.d/00-complianceascode-hardening.conf

LC_ALL=C sed -i "/^\s*RekeyLimit\s\+/Id" "/etc/ssh/sshd_config"
LC_ALL=C sed -i "/^\s*RekeyLimit\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*RekeyLimit\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' "RekeyLimit $var_rekey_limit_size $var_rekey_limit_time" > "/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

- name: XCCDF Value var_rekey_limit_size # promote to variable
  set_fact:
    var_rekey_limit_size: !!str <xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_rekey_limit_size" use="legacy"/>
  tags:
    - always
- name: XCCDF Value var_rekey_limit_time # promote to variable  set_fact:
    var_rekey_limit_time: !!str <xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_var_rekey_limit_time" use="legacy"/>
  tags:
    - always

- name: Force frequent session key renegotiation
  block:

  - name: Deduplicate values from /etc/ssh/sshd_config
    lineinfile:
      path: /etc/ssh/sshd_config
      create: false
      regexp: (?i)^\s*{{ "RekeyLimit"| 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 RekeyLimit is present in /etc/ssh/sshd_config.d
    find:
      paths: /etc/ssh/sshd_config.d
      recurse: 'yes'
      follow: 'no'
      contains: (?i)^\s*{{ "RekeyLimit"| 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)^\s*{{ "RekeyLimit"| 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)^\s*{{ "RekeyLimit"| regex_escape }}\s+
      line: RekeyLimit {{ var_rekey_limit_size }} {{ var_rekey_limit_time }}
      state: present
      insertbefore: BOF
      validate: /usr/sbin/sshd -t -f %s
  when: ansible_virtualization_type not in ["docker", "lxc", "openvz", "podman", "container"]
  tags:
  - configure_strategy
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - sshd_rekey_limit

Force frequent session key renegotiation

Description

Rationale

Remediation - Shell Script

Remediation - Ansible