I would like to suggest another potential security hole in wordpress, but I must apologize in advance for there is no solution against that one: child themes.

The way a child theme works:
- we take one of the php files of that theme
- we kinda "branch it off" into a separate directory, and we apply the changes we desire to this php file
- wordpress process the normal theme normally, except for the theme's php files that have become part of its child theme

Why this is an issue
- themes receive security and functionality updates
- but the vast majority of the users, while they let wordpress update their theme, do not care at all about the php files left behind in the child theme's directory
- and those php files in the child theme directory may contain an exploitable security hole

Let's be real, I never saw someone having a child theme care about checking what changes happen to his/her official theme, and then propagate the changes to his child theme's directory's files.

I don't see at all how that could be prevented.
It's not like we can tell people to go and use file comparison utilities between version x and version x+1 of a theme, and then edit their child theme's php files to manually do the changes. The only persons who understand how it works are already the extremely small minority of persons who keep their child themes up to date.

But, still, that's an actual and big security issue within wordpress.