Disabling access to your compilers is not nearly enough to ensure your servers security. I would suggest googling something like dedicated server security to get a better starting point for the new machine.
In most cases that I have seen access to a compiler was not needed. Users can build the binary on a local machine setup the same as your server and then download the new binary. This is if they go this route in the first place. Most of the time it will be something more along the lines of insecure php/perl scripts hosted on your machine that end up giving access.
Usuing a cgi-telnet script or any script running as priv' user could result in the installation of a rootkit or other means of a backdoor. All it would take is a wget commend in the script itself to get and download something remotely, then compile and run it in the system or simply wget the already compiled binary as mentioned. You may want to google chkrootkit or rkhunter, install one of them and scan the server. It would also be in your best interest to have a professional security auditor audit your server for security holes, and help patch them, then again re-scan the server to make sure everything is fine.
How could my server hack while i have disabled Compilers for unprivileged users
Easily. Compilers are only half the battle. You then have to worry about executables, and the like.
Security is NEVER as simple as one or two tutorials. It's a day to day trick , and it keeps experts on their toes, because there's always new stuff coming up daily.
Know your server, know what's going on with it at all times, know what it's telling you, read the logs, understand them, and you'll find yourself in a whole lot better place for security. Better yet, find yourself a competent admin to take a look @ things for you.
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