cis-ubuntu1204-v110-7-2-2

name	cis-ubuntu1204-v110-7-2-2
description	Disable ICMP Redirect Acceptance
category	cis-os-hardening
version	1.1.0
author	cyberstrike-official
tags	["cis","ubuntu",12.04,"linux","networking","icmp","redirects","sysctl","host-router"]
cis_id	7.2.2
cis_benchmark	CIS Ubuntu 12.04 LTS Server Benchmark v1.1.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

7.2.2 Disable ICMP Redirect Acceptance (Scored)

Profile Applicability

• Level 1

Description

ICMP redirect messages are packets that convey routing information and tell your host (acting as a router) to send packets via an alternate path. It is a way of allowing an outside routing device to update your system routing tables. By setting net.ipv4.conf.all.accept_redirects to 0, the system will not accept any ICMP redirect messages, and therefore, won't allow outsiders to update the system's routing tables.

Rationale

Attackers could use bogus ICMP redirect messages to maliciously alter the system routing tables and get them to send packets to incorrect networks and allow your system packets to be captured.

Audit Procedure

Using Command Line

Perform the following to determine if ICMP redirect messages will be rejected.

/sbin/sysctl net.ipv4.conf.all.accept_redirects
/sbin/sysctl net.ipv4.conf.default.accept_redirects

Expected Result

net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0

Remediation

Using Command Line

Set the net.ipv4.conf.all.accept_redirects and net.ipv4.conf.default.accept_redirects parameters to 0 in /etc/sysctl.conf:

net.ipv4.conf.all.accept_redirects=0
net.ipv4.conf.default.accept_redirects=0

Modify active kernel parameters to match:

/sbin/sysctl -w net.ipv4.conf.all.accept_redirects=0
/sbin/sysctl -w net.ipv4.conf.default.accept_redirects=0
/sbin/sysctl -w net.ipv4.route.flush=1

Default Value

ICMP redirect acceptance is enabled by default.

References

• CIS Ubuntu 12.04 LTS Server Benchmark v1.1.0

Profile

Level 1 - Scored