Just in time for Halloween, this week’s tip exposes a zombie network you may have seen in airports and hotel lobbies across the country: Free Public Wifi. There is actually nothing free, public or even Wifi about this network. It is a rouge, default network that is spread from computer to computer, thanks to an old Windows XP bug. When selected, you will not be able to access the Internet, but you could be exposing your computer to a clever hacker. To learn more about how Free Public Wifi has spread and why it’s become so rampant, read National Public Radio.