1

I'm trying to enable the guest session even if it's disabled by default on Ubuntu 18.04.

My lightdm --show-congif gives me this:

   [Seat:*]
A  allow-guest=true
C  greeter-wrapper=/usr/lib/lightdm/lightdm-greeter-session
D  guest-wrapper=/usr/lib/lightdm/lightdm-guest-session
G  user-session=unity
F  greeter-session=unity-greeter
H  xserver-command=X -core

   [LightDM]
B  backup-logs=false

Sources:
A  /usr/share/lightdm/lightdm.conf.d/50-disable-guest.conf
B  /usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf
C  /usr/share/lightdm/lightdm.conf.d/50-greeter-wrapper.conf
D  /usr/share/lightdm/lightdm.conf.d/50-guest-wrapper.conf
E  /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
F  /usr/share/lightdm/lightdm.conf.d/50-unity-greeter.conf
G  /usr/share/lightdm/lightdm.conf.d/50-unity.conf
H  /usr/share/lightdm/lightdm.conf.d/50-xserver-command.conf

I've already edited /usr/share/lightdm/lightdm.conf.d/50-disable-guest.conf, with this:

# Disable guest sessions due to them not being confined in systemd
# CVE-2017-8900
# https://bugs.launchpad.net/bugs/1663157
[Seat:*]
allow-guest=true

But for some reason that I can't undestand, I still can't see the guest account on the login screen.

Can anyone give me a hint?

Cheers!

  • 1
    Are you using LightDM? Ubuntu 18.04 uses GDM by default. – Gunnar Hjalmarsson Nov 4 '18 at 15:00
  • If I do cat /etc/X11/default-display-manager I get /usr/sbin/gdm3. I was trying to edit lightdm file because I was trying to follow this instructions that instruct to edit the lightdm file. – Matteo M. Nov 4 '18 at 15:33
  • 1
    I understand, but you need to switch to LightDM too if you want to use the guest session. sudo dpkg-reconfigure lightdm – Gunnar Hjalmarsson Nov 4 '18 at 17:11
  • 1
    You are right, my bad. So please help, me to clarify: Ubuntu 18.04 comes out with gmd3 witch doesn't have a guest session. To enable the guest session, I can switch from gdm3 to lightdm but, in doing that, I expose the machine with this vulnerability. Is this correct? – Matteo M. Nov 11 '18 at 13:52
  • Yes. It's up to you to decide if that is something you can live with or not given your setup. – Gunnar Hjalmarsson Nov 11 '18 at 14:48
2

I'm posting a little recap here, that maybe can help people like me, who are a little bit confused about how to enable guest session on Ubuntu 18.04 LTS.

Ubuntu 18.04 comes with GDM as the default display manager. GDM does not have guest session support.

If you are using Ubuntu 18.04 and you need the guest session, you have to switch the display manager: form GDM to lightdm. But pay attenction, in doing this, you expose your machine with this vulnerability, that at this very day, is still open.

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