Well, I'm not really mad at the person who did it, because the hack just seems to be there to point out security flaws and I believe this to be a major problem of W2S.

PHP-Nuke and phpBB both aren't quite known for their tight security. The only way to counteract hack attempts is to stay up-to-date, which ideally means patching within the same day a security patch comes out. I know it's a lot of work, especially if you have many customizations to maintain, but I would rather do without customizations than to leave a system vulnerable to exploits.

I'm writing this because I believe that the CMS hasn't seen any patches for many months. The PHP-Nuke version here is 5.5 but there's already a version 7.8 available. Both versions 6 and 7 have seen security fixes. That makes it quite likely that version 5.5 has many security issues.