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:

```bash
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:

```bash
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