Hello,

I want to ask, if somebody knows some companies that could do a "security" validation of my forum servers + give me advice on how to maintain and maximize security?

Thank you

What are your server specs?

Do you use cpanel or is it just a dedicated box?

Do you have root access?

Did you go through the default 'how to secure my vb instance' thread on vbcom?

I'll answer thoes for an example.
What are your server specs?
What?
Do you use cpanel or is it just a dedicated box?
I use Cpanel
Do you have root access?
Yes
Did you go through the default 'how to secure my vb instance' thread on vbcom?
I think so.

What do I do?

You can install rootkit scanners, you can go through basic locking down of a system, using a proper firewall, or disabling root for ssh, (so you have to su up). and steps like those. Regular maintenance and package updates are important.
I do not know how 'flexible' a system is when cpanel is installed, it takes over quite a bit.

In addition to the suggestions already made (of which I agree with them all) you could consider changing the default SSH port, ensuring a password is set for MySQL (this is often a security hole with cPanel) and installing various things such as BFD (Brute Force Detection) protection. There's a plethora of things which can be done here.

I would also suggest to firewall mysql port to deny access from outside.