Lockscreen bypassed by connecting external monitor on HDMI

Lenovo T460s Mate 18.04

Observed Behavior: computer has lid closed and in sleep. opening the screen results in password protected lockscreen. However, plugging in external monitor on HDMI bypasses the lockscreen and goes straight to the user’s desktop. Laptop is set to turn off built in screen upon connection to external screen on HDMI rather than “mirror” or “extend”. The “lock screen now” widget in the panel stops working after I bypass the login screen in this manner.

Expected Behavior: it should not be possible to bypass lock-screen without entering password no matter what display is used

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interestingly, when connecting a external screen using mini-display port the monitor resolution is detected improperly (this is trying to use external monitor as primary and only monitor). If trying to extend the desktop, then you get flashing screen with unstable picture.

you should do a bugreport

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That is pretty scary.

what subsystem would you file the bug against? Is it mate-desktop or lightdm? can anyone here reproduce?

You could mention both of them in bug-report, and the development team should figure it out i think.

Bug report filed. Marked as security - so it may not be visible publicly

Same issue here. Lock screen disabled by connecting hdmi monitor. HDMI is set as primary display, with laptop screen off.

Ubuntu-Mate 18.04
kernel 4.15.0-22-generic
lightdm 1.26.0
mate-screensaver 1.20.0

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I filed a bug report. Your should do the same. I marked at as security
vulnerability so it may not be searchable.

What machine do you have? Did you upgrade from 16.04 ? Seems related to this post: Screen Not Locked After Waking Up From Suspend

I filed a bug-report which got the security team interested they said they got a few other complaints.

Thank you for your report.
We have had another report of something like that happening to another user
on a Lenovo T450s.
Can you please provide some additional information to help us decide what
to do about this issue?

We would like the output of the commands:

  1. xrandr # when external monitor is connected
  1. inxi -G # same thing
  1. apt-cache policy mate-screensaver
  1. gsettings list-recursively | grep -i screensaver

Can you also please investigate your log files for any messages related
to mate-screensaver crashing?


The bug report is at the link below – but you wont be able to see it because it was marked as a security bug.


thus you can try to email 1769278[ @at@] bugs.launchpad.net and CC this address: etienne.papegnies [@at@] univ-avignon.fr