Gmod backdoor using net?

I joined a server last night and it was hacked by some people now i understand that you can only get admin with a backdoor addon but is it possible to get it with a net message or something along those lines? I was curious as to how they did this. (i know for a fact the server was not backdoored.)

Was backdoored, owners only wanted 2 sound good

they went to backdoor using the frontdoor, that's how they did it

You don't need a backdoor if the front door is unlocked