Home » Does /usr/sbin/nologin as a login shell serve a security purpose?

Does /usr/sbin/nologin as a login shell serve a security purpose?

Solutons:


If you take a look at the nologin man page you’ll see the following description.

excerpt

nologin displays a message that an account is not available and exits
non-zero. It is intended as a replacement shell field to deny login access
to an account.

If the file /etc/nologin.txt exists, nologin displays its contents to the
user instead of the default message.

The exit code returned by nologin is always 1.

So the actual intent of nologin is just so that when a user attempts to login with an account that makes use of it in the /etc/passwd is so that they’re presented with a user friendly message, and that any scripts/commands that attempt to make use of this login receive the exit code of 1.

Security

With respect to security, you’ll typically see either /sbin/nologin or sometimes /bin/false, among other things in that field. They both serve the same purpose, but /sbin/nologin is probably the preferred method. In any case they’re limiting direct access to a shell as this particular user account.

Why is this considered valuable with respect to security?

The “why” is hard to fully describe, but the value in limiting a user’s account in this manner, is that it thwarts direct access via the login application when you attempt to gain access using said user account.

Using either nologin or /bin/false accomplishes this. Limiting your system’s attack surface is a common technique in the security world, whether disabling services on specific ports, or limiting the nature of the logins on one’s systems.

Still there are other rationalizations for using nologin. For example, scp will no longer work with a user account that does not designate an actual shell, as described in this ServerFault Q&A titled: What is the difference between /sbin/nologin and /bin/false?.

Definitely it serves a security purpose. For example, look at the below bug filed for a system user who had a shell.

My debian server was compromised due to the daemon account having a
valid login shell and having samba open for internet access. The break
in was made by setting a password remotly via samba for the daemon
account and the logging in through ssh. Some local root exploit was
then used to OWN my server.

I would recommend you to read this wonderful answer by Gilles where he has provided links to some of the bugs as well.

There are bugs filed over this issue in Debian (274229, 330882,
581899), currently open and classified as “wishlist”. I tend to agree that these are bugs and system users should have /bin/false as their
shell unless it appears necessary to do otherwise.

To add to the excellent answers of @slm and @ramesh:

Yes, as you have pointed out, you can still switch to users with nologin as their default shell by running sudo with a shell defined, but in this case, you have had to:

  1. Log in as another user that has a valid shell
  2. Have sudo permissions configured for that user to run the su command, and
  3. Had your su attempt logged to the sudoers log (assuming of course that sudo logging is enabled).

The users that have nologin defined as their default shell often have higher privileges/are able to do more damage to the system than a regular user, so having them unable to log in directly attempts to limit the damage that a breach of your system could suffer.

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