How dangerous is it to allow users to upload any files to the server?

Question

Malware

There is a site on ubuntu. It is possible for all registered users to upload any type of files. The site is located on the same server where the files are stored.
How dangerous can it be and how can you protect yourself from malicious files?

Answer 1

Very dangerous - a hacker will conquer the server 100%. Let's say you have a PHP website. The user can upload a script to the server and then execute it.
The simplest code will delete the entire site. Do not run yourself!
@unlink($_SERVER["DOCUMENT_ROOT"]);