For a long time, law enforcement agencies and hackers have been able to track the identity and location of mobile users by setting up fake cellular network towers and tricking their devices to connect to them. Researchers have now found that the same thing can be done much more cheaply with a simple Wi-Fi hotspot.
The devices that pose as cell towers are known in the industry as IMSI catchers, with the IMSI (international mobile subscriber identity) being a unique number tied to a mobile subscriber and stored on a SIM card.