cis-ubuntu1804-v220-3-4-2-7

name	cis-ubuntu1804-v220-3-4-2-7
description	Ensure nftables outbound and established connections are configured
category	cis-networking
version	2.2.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-18.04","networking","firewall","nftables"]
cis_id	3.4.2.7
cis_benchmark	CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0 - Control 3.4.2.7

Description

Configure the firewall rules for new outbound, and established connections.

Rationale

If rules are not in place for new outbound, and established connections all packets will be dropped by the default policy preventing network usage.

Impact

None.

Audit Procedure

Command Line

Run the following commands and verify all rules for established incoming connections match site policy:

nft list ruleset | awk '/hook input/,/}/' | grep -E 'ip protocol (tcp|udp|icmp) ct state'

Output should be similar to:

ip protocol tcp ct state established accept
ip protocol udp ct state established accept
ip protocol icmp ct state established accept

Run the following command and verify all rules for new and established outbound connections match site policy:

nft list ruleset | awk '/hook output/,/}/' | grep -E 'ip protocol (tcp|udp|icmp) ct state'

Output should be similar to:

ip protocol tcp ct state established,related,new accept
ip protocol udp ct state established,related,new accept
ip protocol icmp ct state established,related,new accept

Expected Result

Established incoming connections and new/established outbound connections should be accepted as shown above.

Remediation

Command Line

Configure nftables in accordance with site policy. The following commands will implement a policy to allow all outbound connections and all established connections:

nft add rule inet filter input ip protocol tcp ct state established accept
nft add rule inet filter input ip protocol udp ct state established accept
nft add rule inet filter input ip protocol icmp ct state established accept
nft add rule inet filter output ip protocol tcp ct state new,related,established accept
nft add rule inet filter output ip protocol udp ct state new,related,established accept
nft add rule inet filter output ip protocol icmp ct state new,related,established accept

Default Value

No connection tracking rules exist by default.

References

1. NIST SP 800-53 Rev. 5: CA-9, SC-7
2. CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0

CIS Controls

Version 8

4.4 Implement and Manage a Firewall on Servers - Implement and manage a firewall on servers, where supported.