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We have several user accounts that we create for automated tasks that require fine-grained permissions, such as file transfer across systems, monitoring, etc.

How do we lock down these user accounts so that these "users" have no shell and are not able to login? We want to prevent the possibility that someone can SSH in as one of these user accounts.

You can use the usermod command to change a user's login shell.

usermod -s /sbin/nologin myuser

or

usermod -s /usr/sbin/nologin myuser

If your OS does not provide /sbin/nologin, you can set the shell to a NOOP command such as /bin/false:

usermod -s /bin/false myuser

Changing the login shell does not necessarily prevent users from authenticating (except in some services that check if the user's shell is mentioned in /etc/shells).

People may still be able to authenticate to the various services that your system provides to unix users, and may still be authorized to perform some actions albeit probably not run arbitrary commands directly.

Changing the shell to /bin/false or /usr/sbin/nologin will only prevent them from running commands on those services that can be used to run commands (console login, ssh, telnet, rlogin, rexec...), so affect authorisation for some services only.

For ssh for instance, that still allows them to do port forwarding.

passwd -l will disable password authentication, but the user may still be allowed to use other authentication methods (like authorized_keys with ssh).

With pam on Linux at least, you can use the pam_shells module to restrict authentication or authorisation to users with an allowed shell (those mentioned in /etc/shells). For ssh, you'll want to do it at authorisation (account) level as for authentication sshd uses pam in addition to other authentication methods (like authorized_keys), or you can do it with sshd_config directives in /etc/ssh/sshd_config (like AllowUsers and friends).

Beware though that adding some restrictions in global pam authorisation will potentially prevent running cron jobs as those users.

You edit the /etc/passwd file and change the users shell from /bin/bash, or /bin/sh to /sbin/nologin

You can use chsh command:

~# chsh myuser

Enter new shell details when requested:

Login Shell [/bin/sh]: /bin/nologin

Or shorter version:

~# chsh myuser -s /bin/nologin

First, disable the password, using passwd -l username.

Also note in the man page for passwd for option -l:

   -l, --lock

       Lock the password of the named account. This option disables a password by changing it to a value which matches no

       possible encrypted value (it adds a ´!´ at the beginning of the password).



       Note that this does not disable the account. The user may still be able to login using another authentication token

       (e.g. an SSH key). To disable the account, administrators should use usermod --expiredate 1 (this set the account's

       expire date to Jan 2, 1970).



       Users with a locked password are not allowed to change their password.