When you look for tutorials on how to install Proxmox, it always stops after the initial installation - which is quite dangerous, since it leaves a Proxmox host being exposed to the internet while listening to Ports 22 and 8006.
Heck, there is not even any SSH-hardening covered - your SSH is typically set to default values, which means: Password-authentication with the root account is perfectly possible. While you can mitigate some of these things by harding SSH or by installing fail2ban (which we will do anyhow), the host still exposes ports 22 and 8006.
For a secure system, these ports (and actually, apart from one single port ANY ports) have to be disabled. The only acceptable management access to the system should either happen via a Jumpserver or - if not available - via a direct VPN-connection.
In this gist, you find all the required steps to secure your Proxmox exposed host. This gist should work for any Proxmox host with - say - Hetzner, OVH or other vendor