IMSI catcher (fake base station)
A device that impersonates a mobile network's base station so that nearby phones connect to it, exposing subscriber identities and, if it forces an older standard, possibly traffic.
An IMSI catcher is a fake base station. It poses as part of a legitimate mobile network, and nearby phones may connect to it. Its name comes from the International Mobile Subscriber Identity, the permanent identifier on the SIM, which the device can ask a phone to reveal, letting an attacker track which subscribers are present. It is the cellular counterpart of an evil twin on Wi-Fi.
The attack works best against older standards. In 2G the network does not authenticate itself to the phone, so a fake base station can hold the connection and, where encryption is weak or switched off, intercept calls and messages. Forcing a downgrade is therefore a commonly taught technique. 4G LTE already provides mutual authentication between the SIM and the network, though identity requests can still expose the IMSI, and 5G is designed to conceal the permanent identity on the air, where the operator enables this. Cellular encryption, where enabled, protects only the radio link between handset and base station, and cellular authentication proves the SIM, not the person holding the phone.
Exam relevance: a scenario is likely to describe phones in one area being tracked or dropped to an older network. Candidates are expected to see end-to-end protection, such as TLS or a VPN, as the reliable control rather than trust in the cellular link.