Is Your Android Device Telling the World Where You've Been?

Do you own an Android device? Is it less than three years old? If so, then when your phone’s screen is off and it’s not connected to a Wi-Fi network, there's a high risk that it is broadcasting your location history to anyone within Wi-Fi range that wants to listen.

This location history comes in the form of the names of wireless networks your phone has previously connected to. This data is arguably more dangerous than that leaked in previous location data scandals because it clearly denotes in human language places that you've spent enough time to use the Wi-Fi.

In Android we traced this behavior to a feature introduced in Honeycomb (Android 3.1) called Preferred Network Offload (PNO). PNO is supposed to allow phones and tablets to establish and maintain Wi-Fi connections even when they’re in low-power mode (i.e. when the screen is turned off).

The goal is to extend battery life and reduce mobile data usage, since Wi-Fi uses less power than cellular data. But for some reason, even though none of the Android phones we tested broadcast the names of networks they knew about when their screens were on, many of the phones running Honeycomb or later (and even one running Gingerbread) broadcast the names of networks they knew about when their screens were turned off.