“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. These frequently identify places you’ve been, including homes (“Tom’s Wi-Fi”), workplaces (“Company XYZ office net”), churches and political offices (“County Party HQ”), small businesses (“Toulouse Lautrec’s house of ill-repute”), and travel destinations (“Tehran Airport wifi”). 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. Normally eavesdroppers would need to spend some effort extracting this sort of information from the latititude/longitude history typically discussed in location privacy analysis. But even when networks seem less identifiable, there are ways to look them up. We briefly mentioned this problem during our recent post about Apple deciding to randomize MAC addresses in iOS 8. As we pointed out there, Wi-Fi devices that are not actively connected to a network can send out messages that contain the names of networks they’ve joined in the past in an effort to speed up the connection process.1 But after writing that post we became curious just how many phones actually exhibited that behavior, and if so, how much information they leaked. To our dismay we discovered that many of the modern Android phones we tested leaked the names of the networks stored in their settings (up to a limit of fifteen).  And when we looked at these network lists, we realized that they were in fact dangerously precise location histories.”