cis-ubuntu2004-v300-3-3-6

name	cis-ubuntu2004-v300-3-3-6
description	Ensure secure icmp redirects are not accepted
category	cis-networking
version	3.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-20.04","kernel-parameter"]
cis_id	3.3.6
cis_benchmark	CIS Ubuntu Linux 20.04 LTS Benchmark v3.0.0
tech_stack	["ubuntu","linux"]
cwe_ids	[]
chains_with	[]
prerequisites	[]
severity_boost	{}

3.3.6 Ensure secure icmp redirects are not accepted (Automated)

Profile

• Level 1 - Server
• Level 1 - Workstation

Description

Secure ICMP redirects are the same as ICMP redirects, except they come from gateways listed on the default gateway list. It is assumed that these gateways are known to your system, and that they are likely to be secure.

Rationale

It is still possible for even known gateways to be compromised. Setting net.ipv4.conf.all.secure_redirects and net.ipv4.conf.default.secure_redirects to 0 protects the system from routing table updates by possibly compromised known gateways.

Impact

None expected.

Audit Procedure

Command Line

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

• net.ipv4.conf.all.secure_redirects is set to 0
• net.ipv4.conf.default.secure_redirects is set to 0

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 If that file is superseded, the parameter is overridden by an incorrect setting later in that file, or in a canonically later file, that "correct" setting will be ignored both by the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 both net.ipv4.conf.all.secure_redirects and net.ipv4.conf.default.secure_redirects set to 0.

Remediation

Command Line

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

• net.ipv4.conf.all.secure_redirects = 0
• net.ipv4.conf.default.secure_redirects = 0

Example:

# printf '%s\n' "net.ipv4.conf.all.secure_redirects = 0" "net.ipv4.conf.default.secure_redirects = 0" >>
/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.conf.all.secure_redirects=0
  sysctl -w net.ipv4.conf.default.secure_redirects=0
  sysctl -w net.ipv4.route.flush=1
}