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cis-ubuntu1204-v110-9-3-13

DevOps & Security
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Limit Access via SSH

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9.3.13 Limit Access via SSH (Scored)

Profile Applicability

  • Level 1

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 comma separated user names. Numeric userIDs 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 comma separated user names. Numeric groupIDs 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 comma separated user names. Numeric userIDs 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 comma separated group names. Numeric groupIDs 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

Using Command Line

To verify the correct SSH setting, run the following command and verify that the output is as shown:

grep "^AllowUsers" /etc/ssh/sshd_config
grep "^AllowGroups" /etc/ssh/sshd_config
grep "^DenyUsers" /etc/ssh/sshd_config
grep "^DenyGroups" /etc/ssh/sshd_config

Expected Result

AllowUsers <userlist>
AllowGroups <grouplist>
DenyUsers <userlist>
DenyGroups <grouplist>

At least one of these directives should be configured.

Remediation

Using Command Line

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

AllowUsers <userlist>
AllowGroups <grouplist>
DenyUsers <userlist>
DenyGroups <grouplist>

Default Value

No user/group access restrictions are configured by default.

References

  • CIS Ubuntu 12.04 LTS Server Benchmark v1.1.0

Profile

Level 1 - Scored