Servers already crashed because of the action described above. If there's no security issue how someone was able to crash them just like that at the first place? I get it that you need to have privileges, you need to pay to get access to server files, and every action executed with privileges will leave a mark, so it all will be easily sorted out, but it doesn't remove the fact that crashing is still possible.
So as long as it's possible, I find it as an issue and if it's not a security issue then what is it?
I don't think you're completely understanding what I am saying. The person who did this was given access to that server. They used their access to use a stress test program on the server. If they were never given access to the server, this wouldn't have been possible. We're not talking about something like server admin here, we're talking FTP access to the machine the server is running on.
Not only is this not a security risk, it's not even an issue at all. Bottom line, don't give strangers direct access to your server. That's all there is to it.