I came across a site which doesn't allow custom mods/plugins to be installed on their servers because of viruses and such. But when I did some research on these types of mods, I couldn't find any reliable sources which can shed some light on what they can do. All they said was to stick to reliable mod/plugin websites and not to download from shady sites.
Can these mods/plugins do anything? Or are they limited to editing game files and/or crashing servers? Can they do anything a normal pc virus can do?
If they are limited to what they can do, what ways are there to counter these viruses?
By mods/plugins I mean any mods and plugins. Be it server-side, client-side, be it spigot, paper, forge, etc.