Question regarding game "hacking"

I am familiar with the basics of online game hack-proofing (fully authoritative server that does all the checking and simulation, and clients only send inputs).

However, I was wondering: can hackers find, for example, whatever method their client uses to receive their character's position from the server, and then modify that method to make themselves travel 3x faster?

I don't know what is and ins't possible when it comes to hacking. It's something I'm very unfamiliar with. I'm bracing for an "anything can be hacked" answer, but I guess I'd like to know if realistically that is something that is commonly done?

 

Yes, but you validate the input serverside to make sure its within a sane value so if they do some input and it doesn't line up with that then you normalize it or kick them.

 

I was talking about if hackers modified the methods that receive messages from the server, not what they send

so if the server tells a client that "now your position should be (0, 0, 10)", can someone modify the client-side method that applies his position and make himself go to (0, 0, 20) instead?

Edit: Crap, I just realized that would be pointless because it wouldn't change anything on the server side. I guess I answered my own question