Users on my computer have all password requesting options enabled, i.e. password is required on waking from suspend, and on returning from blank screen. The screen turns off after few minutes of inactivity and then gets locked.
We want privacy for everybody.

However, it turned out that one can unlock users without entering a password! I found a few scenarios how it can happen (but does not always happen):

  • User A clicks the 'Lock / Switch Account`,
    or moves away from the computer for a time sufficient to lock the screen.
  • Either black screen or greeting with request for a password appears.
    (Black screen when switching users is a known problem, see: here and here. To get out of a black screen, we press Ctrl+ Alt+Fn.)
    Now one would assume that terminal of user A is locked.
  • Somebody press Ctrl+ Alt+Fn, where Fn corresponds to user A.
  • Console of user A opens. Sometimes it is his greeting with request for a password, but quite often it is just his console without request for a password!

Another possibility:

  • User A at his console presses Ctrl+ Alt+Fn, where Fn corresponds to user B, and sometimes console of user B opens without password.

There is one user whose account is much more likely can be accessed this way. I tried to change settings for requesting password on his account in various ways, but it did not affect the problem.

There is something else different with this user. His greeting is not like greeting of other users. All greetings show user name and password field on the left side of the screen, as in here, but greeting of easy-breakable user is different: all of the data in the middle of the screen, in different layout, and with button "Switch User". I do not understand where it defined. Can different greeting be connected to the problem of entering without password?

Can it be a bug? Which component is responsible for requesting passwords?

Display manager and it's configuration:

cat /etc/X11/default-display-manager

sudo cat /etc/lightdm/lightdm.conf

graphics card and driver info:

sudo lshw -C video 
   description: VGA compatible controller
   product: 4th Generation Core Processor Family Integrated Graphics Controller
   vendor: Intel Corporation
   physical id: 2
   bus info: pci@0000:00:02.0
   version: 06
   width: 64 bits
   clock: 33MHz
   capabilities: msi pm vga_controller bus_master cap_list rom
   configuration: driver=i915 latency=0
   resources: irq:44 memory:f0000000-f03fffff memory:e0000000-efffffff ioport:f000(size=64)

It turns out that a similar question has already been asked before on ru.stackoverflow.com, but there it was only about a greeting.
The old bug 878836 is to blame for both problems, for the login without a password and for an unusual greeting.
Currently, the bug has been partially repaired, and it's already impossible to enter without a password. However, now the password is asked twice, but it's better than not asking at all. According to the latest comment on the launchpad (from 2016-04-04), it will be finally fixed after switching to Unity8 + Mir. Which is not relevant for Ubuntu 18.04 that is using Gnome instead Unity.

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