cis-ubuntu1604-v200-3-5-3-2-4

name	cis-ubuntu1604-v200-3-5-3-2-4
description	Ensure iptables firewall rules exist for all open ports
category	cis-networking
version	2.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-16.04","firewall"]
cis_id	3.5.3.2.4
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 3.5.3.2.4

Profile

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

Description

Any ports that have been opened on non-loopback addresses need firewall rules to govern traffic.

Note:

• 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.
• The remediation command opens up the port to traffic from all sources. Consult iptables documentation and set any restrictions in compliance with site policy.

Rationale

Without a firewall rule configured for open ports default firewall policy will drop all packets to these ports.

Audit Procedure

Command Line

Run the following command to determine open ports:

ss -4tuln

Run the following command to determine firewall rules:

iptables -L INPUT -v -n

Verify all open ports listening on non-localhost addresses have at least one firewall rule.

Remediation

Command Line

For each port identified in the audit which does not have a firewall rule establish a proper rule for accepting inbound connections:

iptables -A INPUT -p <protocol> --dport <port> -m state --state NEW -j ACCEPT

References

None

CIS Controls

Version 7

9.2 Ensure Only Approved Ports, Protocols and Services Are Running - Ensure that only network ports, protocols, and services listening on a system with validated business needs, are running on each system.

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.