Hey, my friend recently brought me a HP USB printer. I plugged it in and KDE showed a message that printer is being installed and then, that installation was completed. I was very suprised when I found out that the printer actually works.

Okay, that's cool, but how that could have happened? I would appreciate if my system didn't reconfigure itself without permission. I checked already /usr/share/polkit-1/actions/ but found nothing interesting there.

Which distro?

You could check the following:
lsusb
cups log files
cups web interface

I can't check it because I don't have this printer anymore. But it is something with priviliges most likely. I use Archlinux.

Ha! I wish my printer installed that easily and I appear to have a very similar setup to yours... Just goes to show: you can't get it right for all the people all the time

per userbase http://userbase.kde.org/Printer_Config

USB printers plugged directly into the computer will probably be automatically recognized and installed by a KDE utility that does the configuration. Even if you disconnect a USB printer, it will remain listed in the printer configuration list.

I do not know the process mentioned nor how to control it

Jup, I have already visited that site before asked the question...

In this case, CUPS itself possibly carried out the automatic configuration.
Do you have any details on the type of notification used to inform you about this auto configuration?

Also, was the HP printer assistant application already running on your computer?

I do have HP device manager but it doesn't run in background. The notification looked just like this one:
http://plasma.kde.org/media/plasma_notifications.png
But I'm afraid I don't remember which program sent it, anyway it wasn't anything HP-related for sure, it looked like KDE's own mechanism.
I found this: http://code.google.com/p/cups-autoconfig/
But still, it needs priviliges from PolicyKit (I hope so), but I can't find appropriate rules. Okay, I guess this is no longer a KDE issue.
Thanks!

Okay, many applications can trigger a dialog such as that. It is possible that your distribution changed the Policykit defaults to permit auto configuration to happen without the user needing to provide credentials or permission.

Archlinux? Come on /etc/polkit-1 and /var/lib/polkit-1 don't have any custom configuration apart of mine.

Permissions could also possibly be defined in /usr/share/polkit-1/actions. I am not aware of a KDE component that does printer auto configuration however (I know tools exist for monitoring the printer, and to manually configure it).