Question

Is it possible to create a hidden user in linux?

Asked by: Alex4583

Hello all.
Let me start off by saying that I'm not a Linux expert. But I've been tasked with making an image for a cyber defense competition and one of the requirements is to create a hidden user with its home directory set to root. Is this possible? If so, how can I go about doing that? Setting the userID to a number below 500? Thank you in advance!

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Asked On
2009-11-06 at 08:55:29ID24878485
Tags

Linux

,

Fedora 9

Topics

Programming User Management

,

Linux Setup

,

Fedora Linux

Participating Experts
6
Points
500
Comments
9

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Answers

 

by: legolasthehansyPosted on 2009-11-06 at 09:48:26ID: 25761274

I don't think this is possible.. You can restrict users on application usage/restrict login to certain servers and the like..

 

by: it4sohoPosted on 2009-11-06 at 10:31:40ID: 25761696

The short answer is a simple "No". Linux has no mechanism for a hidden user.

UNIX systems cannot have hidden users themselves, however you can use a "outside" naming source (like NIS or a Windows AD server), in which case you may be able to hide the presence of other users from current users.

Linux (aka: UNIX in this instance) keeps ALL local usernames in the file /etc/passwd (note: passwords are no longer stored in this file -- at least not usually). This file is accessed whenever a user does virtually anything similar to an ' ls -l" -- as it is the only "database" for mapping userIDs to userNames. As a result, it MUST be world-readable. (This is why passwords were moved OUT of this file into /etc/shadow decades ago).

I hope this helps....

Dan
IT4SOHO

 

by: B-BytePosted on 2009-11-06 at 10:32:21ID: 25761702

I think it can be hidden, if you protect you /etc/passwd file for users who are you hiding from it will be fine. The problem can't be solved if you have to hide yourself from root. You could try to edit the /etc/passwd to change the homedir.

Let us know if it works.

 

by: small_studentPosted on 2009-11-07 at 06:32:10ID: 25766437

To make a user have root privilege (have id less than  500) there are many ways to do that, simplest way is to edit the /etc/passwd file directly

For example

mark:x:504:505::/home/mark:/bin/bash

change the 504 (userid) and 505 (gid) to 0 and you will have a hidden user, with this when you .su - to mark you will get root and if you try id you will also get root.

The interesting part is that they keep a seperate password in the shadow file , thus having 2 users with root privilege one that it is unknown to the system except if you go look the passwd file your self.
 

 

by: TintinPosted on 2009-11-07 at 11:37:39ID: 25767703

It all depends on your definition of "hidden".  I wouldn't consider adding additional users with uid/gid of 0 being hidden.

 

by: chenry334Posted on 2009-11-08 at 21:45:22ID: 25773608

It would probably be possible to add some other string to your /etc/nsswitch.conf file for the passwd: entry.  Most distro's have 'compat' by default which would look at /etc/passwd.  

As said before it does depend on your definition of 'hidden'.  Are you hiding the fact that the user has root privileges?  Are you hiding the fact that the user account exists - ie: no entry in /etc/passwd or /etc/shadow?

As was said before you can set the uid/gid of any user to 0 and it will have root privileges.  If you're trying to do the ladder you can setup the computer to be a NIS server and point the NIS client at itself.  Create a single user account in the NIS passwd file with uid/gid 0 and then change your nsswitch.conf file to use NIS for passwd too.  However this account would still be visible via 'ypcat passwd', finger, etc.  However it wouldn't be in /etc/passwd.

 

by: Alex4583Posted on 2009-11-08 at 21:46:56ID: 31651110

His solution was close to what I was trying to attempt. However, if a person goes under the User Manger (Systerm -> Administrator -> Users and groups) and unchecks the box that hides system users and groups, the hidden user is listed.

 

by: Alex4583Posted on 2009-11-08 at 22:45:16ID: 25773783

chenry334,
I'm trying to hide the fact that the account exists. I know it's pretty impossible to do, but I was able to get an account, in this case (toor) and have root privileges and not have it's user name show up in the login screen.  

 

by: it4sohoPosted on 2009-11-10 at 09:06:19ID: 25787405

Indeed, you can have a different user show up in the login screen at any time -- but that is FAR from a hidden user.

To "hide" a user this way, you just need to duplicate a userID (any one will do, not just root). Only the FIRST user NAME with that ID will show up as the "recorded" user.

The reason for this is simple -- like most things UNIX, the "name" part is just for us (the dumb humans). The ACTUAL information recorded by the system is the UserID (say, 505). Any time you request something that needs a NAME, the system looks up the UID in /etc/passwd and provides the name listed there -- and when it looks, it stops at the FIRST entry that matches.

Now, that brings up an additional point, a correction, and a warning:

1) If the "name" you're trying to hide is the "full name" of the user, that's just a field in /etc/passwd (the 5th, to be exact). The "Name" field is usually a first & last name, but can be anything -- including nothing.

2) Correction -- system privileged accounts are NOT specifically "less than 500"! In fact, there is ONLY ONE system privileged account -- the ROOT account (or any other account with a UID of 0).

3) Secondary accounts with UIDs of 0 (called root-equivalent accounts) are a VERY BAD IDEA! Any user with an id of 0 can easily and accidentally destroy a system IN A FLASH!

I hope this helps explain a little of what's going on here!

Dan
IT4SOHO

20120131-EE-VQP-002

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