cis-ubuntu1804-v220-3-3-8

name	cis-ubuntu1804-v220-3-3-8
description	Ensure source routed packets 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	3.3.8
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 3.3.8

Description

In networking, source routing allows a sender to partially or fully specify the route packets take through the network. In contrast, non-source routed packets travel a path determined by routers in the network. In some cases, systems may not be routable or reachable from some locations (e.g. private addresses vs. internet routable), and so source routed packets would need to be used.

Rationale

Setting net.ipv4.conf.all.accept_source_route, net.ipv4.conf.default.accept_source_route, net.ipv6.conf.all.accept_source_route and net.ipv6.conf.default.accept_source_route to 0 disables the system from accepting source routed packets. Assume this system was capable of routing packets to the internet, an attacker could be redirected through the system to an internal private address using source routed packets.

Impact

None.

Audit Procedure

Command Line

Run the following commands to verify source routed packets are not accepted:

sysctl net.ipv4.conf.all.accept_source_route
sysctl net.ipv4.conf.default.accept_source_route
sysctl net.ipv6.conf.all.accept_source_route
sysctl net.ipv6.conf.default.accept_source_route

Expected Result

net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv6.conf.all.accept_source_route = 0
net.ipv6.conf.default.accept_source_route = 0

Remediation

Command Line

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

net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv6.conf.all.accept_source_route = 0
net.ipv6.conf.default.accept_source_route = 0

Run the following commands to set the active kernel parameters:

sysctl -w net.ipv4.conf.all.accept_source_route=0
sysctl -w net.ipv4.conf.default.accept_source_route=0
sysctl -w net.ipv6.conf.all.accept_source_route=0
sysctl -w net.ipv6.conf.default.accept_source_route=0
sysctl -w net.ipv4.route.flush=1
sysctl -w net.ipv6.route.flush=1

Default Value

net.ipv4.conf.all.accept_source_route = 0, net.ipv4.conf.default.accept_source_route = 0, net.ipv6.conf.all.accept_source_route = 0, net.ipv6.conf.default.accept_source_route = 0

References

1. NIST SP 800-53 Rev. 5: CM-7, SC-5, SC-7
2. CIS Ubuntu Linux 18.04 LTS Benchmark v2.2.0

CIS Controls