Jeroen

I cant find where to write this????

If you mean this literally, you should open up wp-config.php and add those forementioned lines. That should do the trick!

No. The database was upgraded for 3.5 and you cannot run WordPress 3.4 files with a 3.5 database.

How come? Does the upgrade change that much in the database? Or might one get lucky with a little code/DB hacking?

No, this vulnerability hasn’t been fixed yet. It seems that even the recently released 3.5 has a path disclosure vulnerability.

However, seems only Windows falls victim, so if your blog/site is hosted on Linux/Unix, you should be out of the crosshairs.