What is "imsi-catcher"

An IMSI-catcher is a telephone eavesdropping device used for intercepting mobile phone traffic and tracking movement of mobile phone users. Essentially a "fake" mobile tower acting between the target mobile phone and the service provider's real towers, it is considered a man-in-the-middle (MITM) attack. IMSI-catchers are used in some countries by law enforcement and intelligence agencies, but their use has raised significant civil liberty and privacy concerns and is strictly regulated in some countries such as under the German Strafprozessordnung (StPO / Code of Criminal Procedure). Some countries do not even have encrypted phone data traffic (or very weak encryption), thus rendering an IMSI-catcher unnecessary.