cis-ubuntu1604-v200-3-5-2-8

name	cis-ubuntu1604-v200-3-5-2-8
description	Ensure nftables default deny firewall policy
category	cis-networking
version	2.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-16.04","firewall"]
cis_id	3.5.2.8
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 3.5.2.8

Profile

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

Description

Base chain policy is the default verdict that will be applied to packets reaching the end of the chain.

Rationale

There are two policies: accept (Default) and drop. If the policy is set to accept, the firewall will accept any packet that is not configured to be denied and the packet will continue transversing the network stack.

It is easier to white list acceptable usage than to black list unacceptable usage.

Note: Changing firewall settings while connected over network can result in being locked out of the system.

Impact

If configuring nftables over ssh, creating a base chain with a policy of drop will cause loss of connectivity.

Ensure that a rule allowing ssh has been added to the base chain prior to setting the base chain's policy to drop.

Audit Procedure

Command Line

Run the following commands and verify that base chains contain a policy of DROP.

nft list ruleset | grep 'hook input'

nft list ruleset | grep 'hook forward'

nft list ruleset | grep 'hook output'

Expected Result

type filter hook input priority 0; policy drop;

type filter hook forward priority 0; policy drop;

type filter hook output priority 0; policy drop;

Remediation

Command Line

Run the following command for the base chains with the input, forward, and output hooks to implement a default DROP policy:

nft chain <table family> <table name> <chain name> { policy drop \; }

Example:

nft chain inet filter input { policy drop \; }
nft chain inet filter forward { policy drop \; }
nft chain inet filter output { policy drop \; }

Default Value

accept

References

1. Manual Page nft

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.