Disable SSH TCP Forwarding

An XCCDF Rule

Description

The AllowTcpForwarding parameter specifies whether TCP forwarding is permitted. To disable TCP forwarding, add or correct the following line in /etc/ssh/sshd_config:

AllowTcpForwarding no

Rationale

Leaving port forwarding enabled can expose the organization to security risks and back-doors.

ID: xccdf_org.ssgproject.content_rule_sshd_disable_tcp_forwarding
Severity: Medium
References: 2.2 2.2.6
Updated: 5 days ago

- 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_disable_tcp_forwarding

- name: Disable SSH TCP Forwarding
  block:

  - name: Check for duplicate values
    lineinfile:
      path: /etc/ssh/sshd_config
      create: true
      regexp: (?i)(?i)^\s*AllowTcpForwarding\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*AllowTcpForwarding\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*AllowTcpForwarding\s+
      line: AllowTcpForwarding no
      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_disable_tcp_forwarding

# Remediation is applicable only in certain platforms
if dpkg-query --show --showformat='${db:Status-Status}
' 'kernel' 2>/dev/null | grep -q installed; then

if [ -e "/etc/ssh/sshd_config" ] ; then
        LC_ALL=C sed -i "/^\s*AllowTcpForwarding\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' "AllowTcpForwarding no" > "/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

Disable SSH TCP Forwarding

Description

Rationale

Remediation - Ansible

Remediation - Shell Script