Connecting external monitor while screen is locked reveals desktop

[Thomas]

[Impact]
When the screen is locked, connecting an external monitor causes the lock screen to appear on the new monitor, but not on the existing monitor, revealing the contents of the desktop contents of that monitor. A potential attacker can see the contents, but not interact with them without unlocking the system.

[Test Case]
Use Ubuntu 20.04 with the Regolith desktop environment.
gnome-flashback version: 3.36.3-0ubuntu1
Lock screen, attach second monitor. This behavior has been observed by myself in the Regolith desktop environment (uses i3), where it shows one of the i3 workspaces. There may also be other desktop environments affected.
Expected behavior is obviously the contents of the desktop should remain hidden.

[Regression Potential]
Unknown. However, the patch seems trivial and specific for this issue.

[Other Info]
Original Regolith bug report: https://github.com/regolith-linux/regolith-desktop/issues/455
Upstream gnome-flashback has a fix, see this commit: https://gitlab.gnome.org/GNOME/gnome-flashback/-/commit/cdc534a3b0f4abe4e02a5397466ae094ee3d7fdc
I tested this fix myself (single patch on top of the current gnome-flashback Ubuntu sources) and it solves the issue for me, see discussion on Github.

[Launchpad Janitor (janitor)]

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

[Dmitry Shachnev (mitya57)]

This is now fixed in the development Ubuntu release (gnome-flashback 3.37.3). A fix for stable release (Ubuntu 20.04) will be available a bit later.

[Brian Murray (brian-murray)]

Hello Thomas, or anyone else affected,

Accepted gnome-flashback into focal-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/gnome-flashback/3.36.4-0ubuntu1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-focal to verification-done-focal. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-focal. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

[Thomas (twvd)]

Tested with version 3.36.4-0ubuntu1. Locked screen, attached second monitor and primary monitor now shows a lock screen background rather than the desktop contents.

Ergo: looks fixed to me.

[Launchpad Janitor (janitor)]

This bug was fixed in the package gnome-flashback - 3.36.4-0ubuntu1

---------------
gnome-flashback (3.36.4-0ubuntu1) focal; urgency=medium

  * New upstream bugfix release (LP: #1892628).
    - Update lock screen window on monitor changes (LP: #1891855).
    - Don't lock screen on suspend if automatic screen lock is disabled
      (LP: #1891681).
    - Updated translations (LP: #1882531).

 -- Dmitry Shachnev <email address hidden> Sun, 23 Aug 2020 16:04:59 +0300

[Brian Murray (brian-murray)]

The verification of the Stable Release Update for gnome-flashback has completed successfully and the package is now being released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.