If I lock the screen with a password in KDE 4.3 (of Ubuntu), then 50% of the times while entering the password to unlock it, the black rectangle that was over the screen disappears already immediatly making everything visible while the password isn't even typed yet.

IMHO this bug is a security concern. I mean, it's not just for the visual effect that the screen is blackened here (at least not in Gnome and Windows), if you know what I mean.

Is this a known problem?

Are you able to bypass having to enter the password to be able to interact with the desktop? This looks like you may have the screensaver disabled, and as a result no background splash is set..

I am seeing this problem on one PC also. the screen is locked using keyboard, and it goes blank. Then sometimes the blank disappears and the original screen becomes visible. I cannot re-create 100% I have made the screensaver a mad colour so I should be able to see what is blanking what.

I either have the same or similar problem when resuming from suspend. I have it set to lock on resume but occasionallly when I resume the password box is visible even though the rest of the screen around it is not blackened. Or, the whole screen is visible for a short period first before the password box and blackened screen show up. (In either case I can't access the desktop.)

I see this two, the screen will flash visible occasionally, such as when I move the mouse.

I have the same problem. Fedora 14 and KDE 4.5.3.
Before the password box appear for a second or less appear the screen.
It's really annoying where should we post this bug?

More than two years later this security bug is still present in KDE 4.9.3 on Kubuntu 12.10. Is this supposed to be fixed in the new screen locker for KDE (which moves it into the compositor)? I sure hope so!

From what I can tell, this should be solved in KDE 4.10, assuming you use compositing. Unfortunately, without compositing the design of X makes it quite difficult to implement this totally securely. Note that the security of this is only assured if you do not use legacy screen savers.

As part of it's design, Wayland gives full and absolute power to the window manager, so it will be possible to fully secure it there.