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:

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

B  backup-logs=false

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

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?


  • 1
    Are you using LightDM? Ubuntu 18.04 uses GDM by default. Nov 4, 2018 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, 2018 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 Nov 4, 2018 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, 2018 at 13:52
  • Yes. It's up to you to decide if that is something you can live with or not given your setup. Nov 11, 2018 at 14:48

1 Answer 1


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.

  • Long ago fixed vulnerability... for those still looking at this.
    – Eric
    Jun 29, 2020 at 13:28

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