cis-ubuntu1604-v200-3-3-5

name	cis-ubuntu1604-v200-3-3-5
description	Ensure broadcast ICMP requests are ignored
category	cis-networking
version	2.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-16.04","networking"]
cis_id	3.3.5
cis_benchmark	CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 3.3.5

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.

Audit Procedure

Command Line

Run the following commands and verify output matches:

sysctl net.ipv4.icmp_echo_ignore_broadcasts

grep "net\.ipv4\.icmp_echo_ignore_broadcasts" /etc/sysctl.conf /etc/sysctl.d/*

Expected Result

net.ipv4.icmp_echo_ignore_broadcasts = 1

And the grep command should return:

net.ipv4.icmp_echo_ignore_broadcasts = 1

Remediation

Command Line

Set the following parameters in /etc/sysctl.conf or a /etc/sysctl.d/* file:

net.ipv4.icmp_echo_ignore_broadcasts = 1

Run the following commands to set the active kernel parameters:

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

Default Value

icmp_echo_ignore_broadcasts is enabled (set to 1) by default.

References

1. CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0

CIS Controls

Version 7

5.1 Establish Secure Configurations - Maintain documented, standard security configuration standards for all authorized operating systems and software.

Profile Applicability

• Level 1 - Server
• Level 1 - Workstation

Assessment Status

Automated