Use Only Strong Key Exchange algorithms

An XCCDF Rule

Description

Limit the Key Exchange to strong algorithms. The following line in /etc/ssh/sshd_config demonstrates use of those:

KexAlgorithms

Rationale

Key exchange is any method in cryptography by which cryptographic keys are exchanged between two parties, allowing use of a cryptographic algorithm. If the sender and receiver wish to exchange encrypted messages, each must be equipped to encrypt messages to be sent and decrypt messages received

ID: xccdf_org.ssgproject.content_rule_sshd_use_strong_kex
Severity: Medium
References: Req-2.3

2.2 2.2.7

5.2.15

CCE-92626-1
Updated: 5 days ago

# Remediation is applicable only in certain platforms
if rpm --quiet -q kernel-default; then

sshd_strong_kex='<xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_sshd_strong_kex" use="legacy"/>'

if [ -e "/etc/ssh/sshd_config" ] ; then
    
    LC_ALL=C sed -i "/^\s*KexAlgorithms\s\+/Id" "/etc/ssh/sshd_config"
else
    touch "/etc/ssh/sshd_config"
fi
# make sure file has newline at the end
sed -i -e '$a\' "/etc/ssh/sshd_config"

cp "/etc/ssh/sshd_config" "/etc/ssh/sshd_config.bak"
# Insert at the beginning of the file
printf '%s\n' "KexAlgorithms $sshd_strong_kex" > "/etc/ssh/sshd_config"
cat "/etc/ssh/sshd_config.bak" >> "/etc/ssh/sshd_config"
# Clean up after ourselves.
rm "/etc/ssh/sshd_config.bak"

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

- name: Gather the package facts
  package_facts:
    manager: auto
  tags:
  - CCE-92626-1
  - PCI-DSS-Req-2.3  - PCI-DSSv4-2.2
  - PCI-DSSv4-2.2.7
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_use_strong_kex
- name: XCCDF Value sshd_strong_kex # promote to variable
  set_fact:
    sshd_strong_kex: !!str <xccdf-1.2:sub xmlns:xccdf-1.2="http://checklists.nist.gov/xccdf/1.2" idref="xccdf_org.ssgproject.content_value_sshd_strong_kex" use="legacy"/>
  tags:
    - always

- name: Use Only Strong Key Exchange algorithms
  block:

  - name: Check for duplicate values
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)(?i)^\s*KexAlgorithms\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)(?i)^\s*KexAlgorithms\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)(?i)^\s*KexAlgorithms\s+
      line: KexAlgorithms {{ sshd_strong_kex }}
      state: present
      insertbefore: BOF
      validate: /usr/sbin/sshd -t -f %s
  when: '"kernel-default" in ansible_facts.packages'
  tags:
  - CCE-92626-1
  - PCI-DSS-Req-2.3
  - PCI-DSSv4-2.2
  - PCI-DSSv4-2.2.7
  - low_complexity
  - low_disruption
  - medium_severity
  - no_reboot_needed
  - restrict_strategy
  - sshd_use_strong_kex

Use Only Strong Key Exchange algorithms

Description

Rationale

Remediation - Shell Script

Remediation - Ansible