cis-ubuntu2004-v300-5-1-4

Ensure sshd access is configured (Automated)

Profile Applicability

• Level 1 - Server
• Level 1 - Workstation

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 and verify the output:

# sshd -T | grep -Pi -- '^\h*(allow|deny)(users|groups)\h+\H+'

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

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

Review the list(s) to ensure included users and/or groups follow local site policy.

• IF - Match set statements are used in your environment, specify the connection parameters to use for the -T extended test mode and run the audit to verify the setting is not incorrectly configured in a match block. Example additional audit needed for a match block for the user sshuser:

# sshd -T -C user=sshuser | grep -Pi -- '^\h*(allow|deny)(users|groups)\h+\H+'

Note: If provided, any Match directives in the configuration file that would apply are applied before the configuration is written to standard output. The connection parameters are supplied as keyword=value pairs and may be supplied in any order, either with multiple -C options or as a comma-separated list. The keywords are addr (source address), user (user), host (resolved source host name), laddr (local address), lport (local port number), and rdomain (routing domain).

Expected Result

At least one of: allowusers, allowgroups, denyusers, or denygroups should be configured.

Remediation

Command Line

Edit the /etc/ssh/sshd_config file to set one or more of the parameters above any Include and Match set statements as follows:

AllowUsers <userlist>
 - AND/OR -
AllowGroups <grouplist>

Note:

• First occurrence of a option takes precedence, Match set statements withstanding. If Include locations are enabled, used, and order of precedence is understood in your environment, the entry may be created in a .conf file in a Include directory.
• Be advised that these options are "ANDed" together. If both AllowUsers and AllowGroups are set, connections will be limited to the list of users that are also a member of an allowed group. It is recommended that only one be set for clarity and ease of administration.
• It is easier to manage an allow list than a deny list. In a deny list, you could potentially add a user or group and forget to add it to the deny list.

Default Value

None

References

1. SSHD_CONFIG(5)
2. NIST SP 800-53 Rev. 5: AC-3. MP-2
3. SSHD(8)
4. https://documentation.suse.com/en-us/sles/15-SP6/html/SLES-all/cha-ssh.html

CIS Controls

v8 - 3.3 Configure Data Access Control Lists Configure data access control lists based on a user's need to know. Apply data access control lists, also known as access permissions, to local and remote file systems, databases, and applications.

v7 - 4.3 Ensure the Use of Dedicated Administrative Accounts Ensure that all users with administrative account access use a dedicated or secondary account for elevated activities. This account should only be used for administrative activities and not internet browsing, email, or similar activities.

MITRE ATT&CK Mappings: T1021, T1021.004 | TA0008 | M1018