Desktop contents displayed on resume before lock screen is shown

[Paolo Montrasio]

Following to the suggestion at https://bugs.launchpad.net/ubuntu/+source/gnome-screensaver/+bug/1280300/comments/17 I'm opening this new bug. It's very similar, possibly a duplicate, of #1280300 and #1509711 but apparently we have to open a bug for every desktop environment we use.

I'm on 16.04 with Gnome-Flashback Compiz, XDG_CURRENT_DESKTOP=GNOME-Flashback:Unity

When I resume from sleep, the contents of my desktop (including any open windows, emails, etc) are displayed onscreen briefly before the unlock screen is shown. This potentially allows an attacker to view the contents of a locked screen.

To reproduce:
1) Suspend a machine, e.g. by closing the lid
2) Resume the machine

Expected results:
Upon resume, the first thing shown onscreen is the screensaver unlock screen.

Actual results:
Upon resume, the first thing shown onscreen is the set of open windows that were displayed before the machine was put to sleep. After a second or two, the unlock screen is drawn and you have to enter a password to unlock the machine.

This is reproducible on my system.

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04 LTS
Release: 16.04
Codename: xenial

Maybe the package involved is gnome-screesaver

 $ dpkg -l gnome-screensaver
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-=============================================-===========================-===========================-===============================================================================================
ii gnome-screensaver 3.6.1-7ubuntu4 amd64 GNOME screen saver and locker

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Nybo (renenybo)]

I am also affected. It happens sporadically.

HW: Asus Zenbook UX305CA
SW: Ubuntu 16.04.2

[Timo (timo-tk)]

Happens on Ubuntu 18.04 with Unity, Gnome and Budgie.

Hardware: Asus UX303UA.

[Henry (hubofeverything)]

This might be related:
https://bugzilla.gnome.org/show_bug.cgi?id=753678

it's still active, it been like this since 2015-2016 ish, and other distributions report the same thing.

similar links:
https://bugzilla.redhat.com/show_bug.cgi?id=713640
https://bugzilla.redhat.com/show_bug.cgi?id=1391126
https://github.com/linuxmint/Cinnamon/issues/4324

(I use Ubuntu 18.04 (lightdm with gnome), and I encountered the same problem.)

[Pander (pander)]

This also occurs in Lubuntu 18.10 with xscreensaver, LXQt and sddm