An IMSI catcher is a surveillance device that impersonates a legitimate cell tower, prompting nearby mobile phones to connect to it. Once connected, the IMSI catcher can intercept sensitive data such as phone numbers, call metadata, text messages, and sometimes even the content of communications. Law enforcement and intelligence agencies may use IMSI catchers for investigations, but criminals can also exploit them for unauthorized surveillance. The device gets its name from the International Mobile Subscriber Identity (IMSI) number, a unique identifier for each mobile user, which it collects from connected devices. IMSI catchers raise significant privacy and security concerns because users are generally unaware when their phones are being monitored.