This guide is meant to be a Fast & Easy guide for non-programmers to protect their servers and know how exploits basically work.
Don't use plugins for Firewalling
People tend to use plugins for Firewalling like IPWhitelist, OnlyProxyJoin, etc... when hosting a BungeeCord Network.
This plugins aren't really safe because they are not denying connections completelly to your server and in the worst cases this can be...
Fast & Easy Security guide for Minecraft servers (Bungee/Bukkit)