They can even be sneaky, put the cheat part in the compiler, and every program that is compiled with it has the cheat in it, but it is never shown in the source code.

There is a security practice called reproducible binaries that is meant to help ensure this doesn't happen, but it requires a known good compiler, so you have to trust something, and as we are learning to our sorrow, that isn't all that great an idea when dealing with these people.