cis-ubuntu1804-v220-3-4-3-3-1

name	cis-ubuntu1804-v220-3-4-3-3-1
description	Ensure ip6tables default deny firewall policy
category	cis-networking
version	2.2.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-18.04","networking","firewall","iptables","ipv6"]
cis_id	3.4.3.3.1
cis_benchmark	CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.4.3.3.1

Description

A default deny all policy on connections ensures that any unconfigured network usage will be rejected.

Rationale

With a default accept policy the firewall will accept any packet that is not configured to be denied. It is easier to allow list acceptable usage than to deny list unacceptable usage.

Impact

• Changing firewall settings while connected over network can result in being locked out of the system
• Remediation will only affect the active system firewall, be sure to configure the default policy in your firewall management to apply on boot as well

Audit Procedure

Command Line

Run the following command and verify that the policy for the INPUT, OUTPUT, and FORWARD chains is DROP or REJECT:

ip6tables -L -n

-OR-

Verify IPv6 is disabled: Run the following script. Output will confirm if IPv6 is enabled on the system.

#!/usr/bin/bash
{
  if grep -Pqs '^\h*0\b' /sys/module/ipv6/parameters/disable; then
    echo -e " - IPv6 is enabled on the system"
  else
    echo -e " - IPv6 is not enabled on the system"
  fi
}

Expected Result

Chain INPUT (policy DROP)
Chain FORWARD (policy DROP)
Chain OUTPUT (policy DROP)

Or IPv6 is not enabled on the system.

Remediation

Command Line

IF IPv6 is enabled on your system: Run the following commands to implement a default DROP policy:

ip6tables -P INPUT DROP
ip6tables -P OUTPUT DROP
ip6tables -P FORWARD DROP

Default Value

accept

References

1. NIST SP 800-53 Rev. 5: CA-9, SC-7
2. CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0

CIS Controls

Version 8

4.4 Implement and Manage a Firewall on Servers - Implement and manage a firewall on servers, where supported.

4.5 Implement and Manage a Firewall on End-User Devices - Implement and manage a host-based firewall or port-filtering tool on end-user devices.

Version 7

9.4 Apply Host-based Firewalls or Port Filtering - Apply host-based firewalls or port filtering tools on end systems, with a default-deny rule that drops all traffic except those services and ports that are explicitly allowed.