Light Display Manager as login account [closed]

yesterday I installed Ubuntu 13.10, and since I rebooted my laptop, there is a user account called Light Display Manager as available option on my login screen.

A password is needed for login, but I don't know it, although I tried some.

Because of some answers I modified with sudo power one file as follows (added lightdm):

cat /etc/lightdm/users.conf 
#
# User accounts configuration
#
# NOTE: If you have AccountsService installed on your system, then LightDM will
# use this instead and these settings will be ignored
#
# minimum-uid = Minimum UID required to be shown in greeter
# hidden-users = Users that are not shown to the user
# hidden-shells = Shells that indicate a user cannot login
#
[UserAccounts]
minimum-uid=500
hidden-users=nobody nobody4 noaccess lightdm
hidden-shells=/bin/false /usr/sbin/nologin

→ But it didn't changed anything!

What is this? Why is this there? How can I remove (if it's unnecessary)? Thanks in advance for an answer!

Solution 1:

EDIT: in this particular instance it may be a bug - see "Light Display Manager" is offered as login option. The suggested workaround is to add the lightdm account to the list of hidden-users explicitly by editing your /etc/lightdm/lightdm.conf file i.e.

[UserAccounts]
minimum-uid=500
hidden-users=nobody nobody4 noaccess lightdm
hidden-shells=/bin/false /usr/sbin/nologin

[UserAccounts]
minimum-uid=500

so if you are seeing the lightdm account on the login screen, either this value is set too low, or the lightdm account got created with an unusually high UID - you can check with the id command e.g.

id lightdm

Other *nix flavors may use different conventions for the minimum UID for regular user accounts so it's possible yours differ if the accounts were migrated or were chosen for compatibility with another system. You can see the default values for your own system by looking in the /etc/login.defs file.

Solution 2:

In reality there are many accounts in your system. I'll show you: execute a terminal (xterm is fine) and run cat /etc/passwd... every line is a valid account in your system.
Execute man 5 passwd and man 5 shadow to obtain more (always useful) info.
You should ignore the new account created by the login manager and use your account.
If you are curious run sudo less /etc/shadow to see the real account lines of the modern Unix systems like Ubuntu; in this file each line is an account with a crypted password, but the file can be read only by root, so is more secure.
Find the login manager account and observe the second camp: if it's "*" or "!", it's all ok, because an user can't use it (you are also an user :) ) and your PC is relatively secure.
Just a note: if there is a crypted password (unreadable chars) in the new account you are warned as an intruder may have created a false account for a backdoor access, but I think this isn't the case.
Good evening and have a nice experience.

Solution 3:

Looks like a bug in 13.10 (3.11.0-12 kernel) regardless of the minimum-uid - as stated in the referenced bug ticket, lightdm can be added to /etc/lightdm/users.conf as in:

hidden-users=nobody nobody4 noaccess lightdm