IMSI (International Mobile Subscriber Identity).

A 15-digit identifier stored on the SIM or eSIM profile that uniquely identifies a subscriber on a cellular network, made of MCC, MNC, and MSIN fields. It belongs to the Mobile Security category of cybersecurity.

A 15-digit identifier stored on the SIM or eSIM profile that uniquely identifies a subscriber on a cellular network, made of MCC, MNC, and MSIN fields.

The IMSI is the subscriber identity used by GSM, UMTS, LTE, and 5G to look up subscriber records in the operator's HLR/HSS/UDM. In older networks, the IMSI was sometimes sent over the air in cleartext during attach, which enabled IMSI catchers (also called Stingrays) — devices that impersonate a base station to collect IMSIs and even downgrade traffic to weakly encrypted 2G. 5G addresses this by using SUPI/SUCI: the permanent identifier is hidden behind a public-key encryption of the IMSI. Defences against IMSI catchers on mobile devices include strong baseband isolation, monitoring tools like Android's AIMSICD or research apps, disabling 2G when possible, and using end-to-end encrypted apps so the cellular layer carries less sensitive content.

Defences for IMSI (International Mobile Subscriber Identity) typically combine technical controls and operational practices, as detailed in the full definition above.

Common alternative names include: IMSI, Subscriber identity.