cis-ubuntu2004-v300-4-4-2-4

name	cis-ubuntu2004-v300-4-4-2-4
description	Ensure iptables firewall rules exist for all open ports
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","firewall","iptables"]
cis_id	4.4.2.4
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 4.4.2.4

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.

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
• 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

Netid  State      Recv-Q Send-Q    Local Address:Port         Peer Address:Port
udp    UNCONN     0      0                    *:68                       *:*
udp    UNCONN     0      0                    *:123                      *:*
tcp    LISTEN     0      128                  *:22                       *:*

Run the following command to determine firewall rules:

iptables -L INPUT -v -n

Chain INPUT (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0
    0     0 DROP       all  --  *      *       127.0.0.0/8          0.0.0.0/0
    0     0 ACCEPT     tcp  --  *      *       0.0.0.0/0            0.0.0.0/0            tcp dpt:22 state NEW

Verify all open ports listening on non-localhost addresses have at least one firewall rule. The last line identified by the tcp dpt:22 state NEW identifies it as a firewall rule for new connections on tcp port 22.

Expected Result

All open ports on non-localhost addresses have corresponding iptables rules.

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

Default Value

Not applicable.