I think I speak for every network manager and privacy advocate in the world when I say, “fuck you with a rusty crowbar”.
For those who don’t know, one of the features in the Windows 10 beta (and already in the field in Windows Phone 8.1) is WiFi Sense. The short version is, if you share your wireless access with someone, you’re now potentially sharing it with everyone on their contacts list from Outlook.com, Skype, and even Facebook if they link their accounts.
And the network owner can’t stop them from sharing the password, or even find out that it’s happened. MS offers only one way to prevent this from happening, and that’s changing your network’s SSID to contain the string “_optout”. (This article notes that Google has their own magic string to prevent your wireless from being mapped by their cars, so the new hotness is “_optout_nomap”. No doubt Apple will jump on the bandwagon as well, and next year it will have to be “_optout_nomap_nocandyfromstrangers”).
They claim it will only give limited access to all these strangers, and not let them see anything else that’s on your home network, but that requires that we not only believe that there are no security holes in a Microsoft product, but that the raw password is securely stored in three different online services and every stranger’s device.
The only real defense is to use WPA2 Enterprise authentication, which requires a Radius server. Unfortunately, a lot of consumer-grade wireless-only products won’t do that at all. Last time I tried to get a Kindle to use it, it detected it but never actually sent the username/password combination.
[Update: Microsoft’s FAQ for this misfeature includes the statement:
It can take several days for your network to be added to the opted-out list for Wi-Fi Sense. If you want to stop your network from being shared sooner than that, you can change your Wi-Fi network password.
Crowbar, Rusty. Rinse and repeat.]
[Update: just tested a Kindle Paperwhite against a WPA2 Enterprise wireless running TTLS/PAP user-based authentication. It sent an empty password, so no, you can’t protect your home wireless from Wifi Sense if you plan to connect common small devices to it.]