IMSI (International Mobile Subscriber Identity)

The International Mobile Subscriber Identity is a globally unique number, up to 15 digits, that identifies a subscriber within a mobile network. It is composed of a Mobile Country Code, Mobile Network Code, and Mobile Subscriber Identification Number, and is stored on the SIM. Because the IMSI is the long-term identity used for authentication and routing, exposing it enables targeted surveillance and is the goal of IMSI catchers and many SS7/Diameter attacks. 5G replaces over-the-air IMSI exposure with the encrypted SUCI to mitigate this.

Categories: SignalingIdentity and SubscriberSecurity Controls

IMSI in context

Signaling protocols carry the control-plane messages that set up calls, register subscribers and route SMS. SS7, Diameter, GTP-C and SIP are the four dominant families, and interconnect exposure of any of them is treated by GSMA as top-tier telecom risk.

Subscriber identifiers (IMSI, SUPI, MSISDN, IMEI) anchor authentication, charging and lawful interception. Any protocol that leaks a permanent identifier is treated as a privacy defect, which is why 5G introduced SUCI concealment on the air interface.

To place IMSI in the wider telecom-security picture, review AAA, Any-Time Interrogation (ATI), CAP, STIR, RCS and 1G — each entry cross-references back to this page so you can walk the topic in either direction.

Related terms

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