cis-ubuntu2004-v300-3-3-4

name	cis-ubuntu2004-v300-3-3-4
description	Ensure broadcast icmp requests are ignored
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","kernel-parameter"]
cis_id	3.3.4
cis_benchmark	CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

3.3.4 Ensure broadcast icmp requests are ignored (Automated)

Profile

• Level 1 - Server
• Level 1 - Workstation

Description

Setting net.ipv4.icmp_echo_ignore_broadcasts to 1 will cause the system to ignore all ICMP echo and timestamp requests to broadcast and multicast addresses.

Rationale

Accepting ICMP echo and timestamp requests with broadcast or multicast destinations for your network could be used to trick your host into starting (or participating) in a Smurf attack. A Smurf attack relies on an attacker sending large amounts of ICMP broadcast messages with a spoofed source address. All hosts receiving this message and responding would send echo-reply messages back to the spoofed address, which is probably not routable. If many hosts respond to the packets, the amount of traffic on the network could be significantly multiplied.

Impact

None expected.

Audit Procedure

Command Line

Run the following script to verify the following kernel parameter is set in the running configuration and correctly loaded from a kernel parameter configuration file:

• net.ipv4.icmp_echo_ignore_broadcasts is set to 1

Note: kernel parameters are loaded by file and parameter order precedence. The following script observes this precedence as part of the auditing procedure. The parameters being checked may be set correctly in a file. If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the script and by the system during a normal kernel parameter load sequence.

Expected Result

Audit Result: ** PASS ** with net.ipv4.icmp_echo_ignore_broadcasts set to 1.

Remediation

Command Line

Set the following parameter in /etc/sysctl.conf or a file in /etc/sysctl.d/ ending in .conf:

• net.ipv4.icmp_echo_ignore_broadcasts = 1

Example:

# printf '%s\n' "net.ipv4.icmp_echo_ignore_broadcasts = 1" >> /etc/sysctl.d/60-netipv4_sysctl.conf

Run the following script to set the active kernel parameters:

#!/usr/bin/env bash

{
  sysctl -w net.ipv4.icmp_echo_ignore_broadcasts=1
  sysctl -w net.ipv4.route.flush=1
}

Note: If these settings appear in a canonically later file, or later in the same file, these settings will be overwritten.