A group of academics at the University of Hamburg (Germany) demonstrated that it is possible to use WiFi connection probe requests to identify and track devices and thereby their users.
Mobile devices transmit probe requests to receive information about nearby Wi-Fi networks and establish a Wi-Fi connection. An access point receiving a probe request replies with a probe response, thereby establishing a connection between both devices. However, probe requests can contain identifying information about the device owner depending on the age of the device and its OS. For example, a request can contain the preferred network list (PNL), which includes networks identified by their so-called Service Set Identifier (SSIDs). Experts explained that 23 % of the probe requests contain SSIDs of networks the devices were connected to in the past.