cis-ubuntu1604-v200-3-5-3-2-1

name	cis-ubuntu1604-v200-3-5-3-2-1
description	Ensure iptables default deny firewall policy
category	cis-networking
version	2.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-16.04","firewall"]
cis_id	3.5.3.2.1
cis_benchmark	CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 3.5.3.2.1

Profile

• Level: Level 1 - Server, Level 1 - Workstation
• Assessment Status: Automated

Description

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

Notes:

• 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.

Rationale

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

Audit Procedure

Command Line

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

iptables -L

Expected Result

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

Remediation

Command Line

Run the following commands to implement a default DROP policy:

iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP

References

None

CIS Controls

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.