cis-ubuntu1604-v200-5-3-5

name	cis-ubuntu1604-v200-5-3-5
description	Ensure SSH access is limited
category	cis-networking
version	2.0.0
author	cyberstrike-official
tags	["cis","ubuntu","linux","ubuntu-16.04","ssh","remote-access"]
cis_id	5.3.5
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 5.3.5

Description

There are several options available to limit which users and group can access the system via SSH. It is recommended that at least one of the following options be leveraged:

• AllowUsers: The AllowUsers variable gives the system administrator the option of allowing specific users to ssh into the system. The list consists of space separated user names. Numeric user IDs are not recognized with this variable. If a system administrator wants to restrict user access further by only allowing the allowed users to log in from a particular host, the entry can be specified in the form of user@host.
• AllowGroups: The AllowGroups variable gives the system administrator the option of allowing specific groups of users to ssh into the system. The list consists of space separated group names. Numeric group IDs are not recognized with this variable.
• DenyUsers: The DenyUsers variable gives the system administrator the option of denying specific users to ssh into the system. The list consists of space separated user names. Numeric user IDs are not recognized with this variable. If a system administrator wants to restrict user access further by specifically denying a user's access from a particular host, the entry can be specified in the form of user@host.
• DenyGroups: The DenyGroups variable gives the system administrator the option of denying specific groups of users to ssh into the system. The list consists of space separated group names. Numeric group IDs are not recognized with this variable.

Rationale

Restricting which users can remotely access the system via SSH will help ensure that only authorized users access the system.

Audit Procedure

Command Line

Run the following command:

sshd -T -C user=root -C host="$(hostname)" -C addr="$(grep $(hostname) /etc/hosts | awk '{print $1}')" | grep -Ei '^\s*(allow|deny)(users|groups)\s+\S+'

Verify that the output matches at least one of the following lines:

allowusers <userlist>
allowgroups <grouplist>
denyusers <userlist>
denygroups <grouplist>

Remediation

Command Line

Edit the /etc/ssh/sshd_config file to set one or more of the parameter as follows:

AllowUsers <userlist>

OR