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5.1.4 Ensure SSH access is limited

Audit#

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).

Remediation#

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 an 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