You need a special device to use the internet over a power line. This is needed to transmit and receive. From my experience it really depends on the quality of the power lines in your home as well as the electrical noise on the line. If your home was built with cheap power lines or there is noise on the line, it will greatly interfere with your connection.
I have something like:
I no longer use them, because the connection is unstable for the aforementioned reasons. They do have encryption, and you can change the password.
Is this what you are talking about ? They can hack it, if they crack the encryption, but it is unlikely because the signal looses strength over time, and it will be gone by the time it leaves the house.