No password required when fast switch to logged-in user in Gnome 3 (classic)

[Sven Wyes]

http://forum.ubuntuusers.de/topic/benutzerwechsel-ohne-passwort

lsb_release -rd :
Description: Ubuntu 11.10
Release: 11.10

apt-cache policy gnome :
gnome:
  Installiert: (keine)
  Kandidat: 1:3.0+1ubuntu1
  Versionstabelle:
     1:3.0+1ubuntu1 0
        500 http://de.archive.ubuntu.com/ubuntu/ oneiric/universe amd64 Packages

On my PC there are two users: A (admin) and B (normal) plus a guest-account.
A is logged in with the gnome classic desktop. If he clicks 'change user', lightdm is shown. Then user B types his password into the password field and gets onto the unity desktop. There he can click 'change user to A'. For this action no password is required, A's desktop will be shown unlocked.

I expect that a password must be typed in to perform a user change.

Maybe this bug is caused by gnome not locking the screen automatically.

In case of communication difficulties you may see the German explanation of this problem (linked above) or request me for a clarification. I hope the English description is not too poor.