cis-ubuntu1604-v200-5-3-21

name	cis-ubuntu1604-v200-5-3-21
description	Ensure SSH AllowTcpForwarding is disabled
category	cis-networking
version	2.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-16.04","ssh","remote-access"]
cis_id	5.3.21
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 5.3.21

Description

SSH port forwarding is a mechanism in SSH for tunneling application ports from the client to the server, or servers to clients. It can be used for adding encryption to legacy applications, going through firewalls, and some system administrators and IT professionals use it for opening backdoors into the internal network from their home machines.

Rationale

Leaving port forwarding enabled can expose the organization to security risks and back-doors.

SSH connections are protected with strong encryption. This makes their contents invisible to most deployed network monitoring and traffic filtering solutions. This invisibility carries considerable risk potential if it is used for malicious purposes such as data exfiltration. Cybercriminals or malware could exploit SSH to hide their unauthorized communications, or to exfiltrate stolen data from the target network.

Impact

SSH tunnels are widely used in many corporate environments that employ mainframe systems as their application backends. In those environments the applications themselves may have very limited native support for security. By utilizing tunneling, compliance with SOX, HIPAA, PCI-DSS, and other standards can be achieved without having to modify the applications.

Audit Procedure

Command Line

Run the following command and verify that output matches:

sshd -T -C user=root -C host="$(hostname)" -C addr="$(grep $(hostname) /etc/hosts | awk '{print $1}')" | grep -i allowtcpforwarding

Expected Result

allowtcpforwarding no

Run the following command and verify the output:

grep -Ei '^\s*AllowTcpForwarding\s+yes' /etc/ssh/sshd_config

Nothing should be returned.

Remediation

Command Line

Edit the /etc/ssh/sshd_config file to set the parameter as follows:

AllowTcpForwarding no

Default Value

AllowTcpForwarding yes

References

1. https://www.ssh.com/ssh/tunneling/example
2. SSHD_CONFIG(5)

CIS Controls

Version 7

9.2 Ensure Only Approved Ports, Protocols and Services Are Running - Ensure that only network ports, protocols, and services listening on a system with validated business needs, are running on each system.