How does google know where MAC addresses are? I thought that they were non-routable, so they are limited to the extremely local connection. Presumably the range from your computer to the termination of the cable modem on the other end.
Most of the comcast cable modems I've had in the past years let you see the mac address if you navigate to 192.168.100.1 without any password, so xss that loaded that page would work.