IMSI stands for “International Mobile Subscriber Identity”. The technology is essentially a mobile phone tower with “a malicious operator”. It mimics the behavior of a cell tower and tricks mobile phones into sending data to it, instead of to the tower.
As such it is considered a Man In the Middle (MITM) attack. It is used as an eavesdropping device used for interception and tracking of cellular phones and usually is undetectable for the users of mobile phones.
Once it has made a connection with the phone and tricked it into thinking it is a mobile tower, the IMSI catcher forces the phone to drop its encryption, enabling easy access to the contents of the device. The tool then lets the attacker listen in on mobile conversations and intercept all data sent from a mobile phone, remaining undetected. In some cases the tool also allows the operator to manipulate messages.
This could definitely be a big problem!