cis-ubuntu2004-v300-4-2-8

name	cis-ubuntu2004-v300-4-2-8
description	Ensure ufw default deny firewall policy
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","firewall","ufw"]
cis_id	4.2.8
cis_benchmark	CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0 - Control 4.2.8

Profile

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

Description

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

Note: Any port or protocol without a explicit allow before the default deny will be blocked

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

Any port and protocol not explicitly allowed will be blocked. The following rules should be considered before applying the default deny.

ufw allow out http
ufw allow out https
ufw allow out ntp # Network Time Protocol
ufw allow out to any port 53 # DNS
ufw allow out to any port 853 # DNS over TLS
ufw logging on

Audit Procedure

Command Line

Run the following command and verify that the default policy for incoming, outgoing, and routed directions is deny, reject, or disabled:

ufw status verbose | grep Default:

Example output:

Default: deny (incoming), deny (outgoing), disabled (routed)

Expected Result

Default: deny (incoming), deny (outgoing), disabled (routed)

Remediation

Command Line

Run the following commands to implement a default deny policy:

ufw default deny incoming
ufw default deny outgoing
ufw default deny routed

Default Value

Not applicable.

References

1. NIST SP 800-53 Rev. 5: SC-7

CIS Controls

v8 - 4.4 Implement and Manage a Firewall on Servers v8 - 4.5 Implement and Manage a Firewall on End-User Devices v7 - 9.4 Apply Host-based Firewalls or Port Filtering

MITRE ATT&CK Mappings: T1562, T1562.004 | TA0011 | M1031, M1037