cis-ubuntu2004-v300-4-3-6

name	cis-ubuntu2004-v300-4-3-6
description	Ensure nftables loopback traffic is configured
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","firewall","nftables"]
cis_id	4.3.6
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 4.3.6

Profile

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

Description

Configure the loopback interface to accept traffic. Configure all other interfaces to deny traffic to the loopback network

Rationale

Loopback traffic is generated between processes on machine and is typically critical to the operation of the system. The loopback interface is the only place that loopback network traffic should be seen, all other interfaces should ignore traffic on this network as an anti-spoofing measure.

Audit Procedure

Command Line

Run the following commands to verify that the loopback interface is configured: Run the following command to verify the loopback interface is configured to accept network traffic:

nft list ruleset | awk '/hook input/,/}/' | grep 'iif "lo" accept'

Example output:

iif "lo" accept

Run the following command to verify network traffic from an IPv4 loopback interface is configured to drop:

nft list ruleset | awk '/hook input/,/}/' | grep 'ip saddr'

Example output:

ip saddr 127.0.0.0/8 counter packets 0 bytes 0 drop

• IF - IPv6 is enabled on the system: Run the following command to verify network traffic from an IPv6 loopback interface is configured to drop:

nft list ruleset | awk '/hook input/,/}/' | grep 'ip6 saddr'

Example output:

ip6 saddr ::1 counter packets 0 bytes 0 drop

Expected Result

Loopback accept rule exists, and drop rules for 127.0.0.0/8 and ::1 are present.

Remediation

Command Line

Run the following commands to implement the loopback rules:

nft add rule inet filter input iif lo accept
nft add rule inet filter input ip saddr 127.0.0.0/8 counter drop

• IF - IPv6 is enabled on the system: Run the following command to implement the IPv6 loopback rule:

nft add rule inet filter input ip6 saddr ::1 counter drop