All PHP-Nuke versions, including the just released 6.0, are vulnerable to a very simple SQL injection that may lead to a basic DoS attack.
For instance, if you create a short script, to send a few requests, (I have tested with just 6) similar to this:
after a real short time the load of the machine is so high that it will become inacessible. When the script is stopped, the server will take a few minutes to recover from the load and become acessible again.
Well, the number of requests depends on your MySQL parameters and hardware, but in general all the tested php-nuke sites where vulnerable and become inacessible.
If you are running PHP-Nuke, I suggest the creation of some filters to avoid this kind of attack. Other things can be made, but I will not talk about them now. I will wait until Francisco fix them.
Francisco was noticed a month ago, but the problems persist. Maybe he is busy reading the new revision of the "Building Secure Web Applications and Web Services" OWASP document. :]