Hiding Shutdown/reboot in logout dialog and possible security issue

[Anders Bruun Olsen]

Hi,

Well, the problem is that lightdm runs as root and happily takes commands
from any user logged in. This might be quite handy for desktop machines,
but it isn't pretty from a security perspective :)
Anyways, I don't need a login manager at all, since Nomachine just spins up
an lxsession for users. Nobody needs to login locally on the machine.

When users try to use the shutdown/reboot buttons they are given a
username/password dialog box, and when they can't provide a username with
sudo privileges they get an "access denied" message, which is fine. It
would just be better if I could hide those buttons completely so no users
get confused about what to push to logout. Admins that actually need to
reboot or shutdown the machine will do it from a prompt anyways.


2014-07-10 14:49 GMT+02:00 Israel <israeldahl at gmail.com>:

>  On 07/10/2014 07:21 AM, Anders Bruun Olsen wrote:
>
> Hi,
>
>  I am looking to build a new terminal server for remote desktops which
> will be accessed through NoMachine Enterprise. Lubuntu and LXDE looks like
> a nice fit, but I have run into a couple of issues.
>
>  I have a default install of Lubuntu 14.04 64-bit. I have created a
> non-privileged user (no sudo rights). I have also installed NoMachine
> Enterprise Server. First thing I discovered was what I would almost call a
> security issue. When my non-privileged user is logged in remotely (with
> Nomachine Enterprise Client), choosing shutdown in the logout dialog
> actually does shut down the server. How can this user shut down the server,
> without root access? I found out, that if I ensure lightdm isn't running
> (nobody will login locally), my unprivileged user can't shut down the
> server, but will be asked for the password to a privileged user, so I guess
> this is an issue with lightdm. Is this really intended behavior?
>
>  Next up, I would like to hide the shutdown and reboot buttons in the
> logout dialog. The only way I have been able to find by searching, is to
> actually change the source code for lxsession-logout and recompile. Is
> there really no other way to hide those buttons?
>
>  --
> Anders Bruun Olsen
> It-ansvarlig
> Det Danske Sprog- og Litteraturselskab
> (Society for Danish Language and Literature)
>
>
>  Hi Anders!
> The commands for shutting down, and rebooting are sudo commands. (sudo
> shutdown -h now or sudo reboot)  However certain programs are granted
> access to run these commands from root.  So it should be possible to not
> allow those programs to shutdown without a password.  This also includes
> the lxsession-logout.  Can the unprivaleged user run the logout dialog and
> shutdown from there as well (this is the power button on the panel, unless
> you have removed it).
>
> Unfortunately I cannot remember right this moment what grants these
> programs access to run those commands without need a user password.  But
> this might give you something to search for.  There are some people on this
> list who may be able to clarify what I am getting at, and may even know how
> this is done specifically.
>
> On the other hand, there are other login managers such as SLIM.  You may
> need to configure it a bit to get it looking the way you want and have the
> options you want, but that might be the easier choice, unless the logout
> dialog can let the user shutdown the server as well...
>
> --
> Regards
>
>
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>


-- 
Anders Bruun Olsen
It-ansvarlig
Det Danske Sprog- og Litteraturselskab
(Society for Danish Language and Literature)
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