cis-ubuntu1804-v220-3-3-5

name	cis-ubuntu1804-v220-3-3-5
description	Ensure ICMP redirects are not accepted
category	cis-networking
version	2.2.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-18.04","networking","kernel-parameter"]
cis_id	3.3.5
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 3.3.5

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 and net.ipv6.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's packets to be captured.

Impact

None.

Audit Procedure

Command Line

Run the following script to verify ICMP redirects are not accepted:

#!/usr/bin/bash
{
  l_output="" l_output2=""
  a_parlist=("net.ipv4.conf.all.accept_redirects=0" "net.ipv4.conf.default.accept_redirects=0" "net.ipv6.conf.all.accept_redirects=0" "net.ipv6.conf.default.accept_redirects=0")

  for l_kpe in "${a_parlist[@]}"; do
    l_kpname="$(awk -F= '{print $1}' <<< "$l_kpe")"
    l_kpvalue="$(awk -F= '{print $2}' <<< "$l_kpe")"
    l_krp="$(sysctl "$l_kpname" | awk -F= '{print $2}' | xargs)"
    if [ "$l_krp" = "$l_kpvalue" ]; then
      l_output="$l_output\n - \"$l_kpname\" is correctly set to \"\""
    
      l_output2=
    
  

   [ -z  ]; 
     -e 
  
     -e 
    [ -n  ] &&  -e 
  
}