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 instructions that set up calls, register subscribers, hand over sessions 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 FS.11 / FS.19 / FS.20 as a top-tier telecom risk.

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

To place IMSI in the wider telecom-security picture, review RCS, Any-Time Interrogation (ATI), IAMF, IMSI Catcher (Cell Site Simulator), 1G and 2G.

Related terms

Related topic hubs

  • Diameter Attacks S6a IDR/PUR/CLR abuse, subscriber tracking, LTE signaling attacks.
  • Signaling Firewall Unified SS7/Diameter/GTP/SMS signaling firewall framework.
  • 5G Security 5G SBA, SEPP, SUCI, 5G-AKA, N32 and service-based interface security.
  • 5G NEF Security Network Exposure Function security in the 5G core.

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