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

name	cis-ubuntu1804-v220-3-4-3-2-2
description	Ensure iptables loopback traffic is configured
category	cis-networking
version	2.2.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-18.04","networking","firewall","iptables"]
cis_id	3.4.3.2.2
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 3.4.3.2.2

Description

Configure the loopback interface to accept traffic. Configure all other interfaces to deny traffic to the loopback network (127.0.0.0/8).

Rationale

Loopback traffic is generated between processes on machine and is typically critical to operation of the system. The loopback interface is the only place that loopback network (127.0.0.0/8) traffic should be seen, all other interfaces should ignore traffic on this network as an anti-spoofing measure.

Impact

• 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

Audit Procedure

Command Line

Run the following commands and verify output includes the listed rules in order (packet and byte counts may differ):

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

iptables -L OUTPUT -v -n

Chain OUTPUT (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

Expected Result

The INPUT chain should accept all traffic on the loopback interface and drop all traffic from 127.0.0.0/8. The OUTPUT chain should accept all traffic going out the loopback interface.

Remediation

Command Line

Run the following commands to implement the loopback rules:

iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
iptables -A INPUT -s 127.0.0.0/8 -j DROP

Default Value

No loopback rules are configured 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.

4.5 Implement and Manage a Firewall on End-User Devices - Implement and manage a host-based firewall or port-filtering tool on end-user devices.

Version 7