Cross-device tracking refers to technology which enables tracking of users across multiple devices, such as televisions, smartphones, and personal computers. Tracking users across multiple devices is made possible using inaudible sounds made by one device and picked up by the microphone of another device; these inaudible sounds are called "audio beacons".
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It is possible that cross-device tracking could be used to confirm the identities of users on the Tor anonymity network.
Background
In October 2015, the Center for Democracy and Technology submitted comments to the Federal Trade Commission (FTC) regarding cross-device tracking technology, specifically mentioning SilverPush.
Audio "beacons" can be embedded into television advertisements; these beacons can be picked up by mobile apps, such as those using the SilverPush SDK. This allows the behavior of users to be tracked, including which ads were seen by the user and how long they watched an ad before changing the channel.
In March 2016, the FTC issued warning letters to 12 app developers using cross-device tracking in their apps. The FTC warned these developers that they may be violating the FTC Act if they state or imply that their apps are not tracking television viewing habits when they in fact are.
Privacy implications
Cross-device tracking has privacy implications and allows for more detailed tracking of users than traditional tracking methods. Data can be collected from multiple devices used by a single user and correlated to form a more accurate picture of the person being tracked.