'U.S. military officials were recently caught off guard by revelations that servicemembers’ digital fitness trackers were storing the locations of their workouts – including at or near military bases and clandestine sites around the world. But this threat is not limited to Fitbits and similar devices. My group’s recent research has shown how mobile phones can also track their users through stores and cities and around the world – even when users turn off their phones’ location-tracking services.
The vulnerability comes from the wide range of sensors phones are equipped with – not just GPS and communications interfaces, but gyroscopes and accelerometers that can tell whether a phone is being held upright or on its side and can measure other movements too. Apps on the phone can use those sensors to perform tasks users aren’t expecting – like following a user’s movements turn by turn along city streets.
Most people expect that turning their phone’s location services off disables this sort of mobile surveillance. But the research I conduct with my colleagues Sashank Narain, Triet Vo-Huu, Ken Block and Amirali Sanatinia at Northeastern University, in a field called “side-channel attacks,” uncovers ways that apps can avoid or escape those restrictions. We have revealed how a phone can listen in on a user’s finger-typing to discover a secret password – and how simply carrying a phone in your pocket can tell data companies where you are and where you’re going.'