What is International Mobile Subscriber Identity (IMSI)?

The International Mobile Subscriber Identity (IMSI) is a unique numeric identifier assigned to each mobile network subscriber and stored on the subscriber’s SIM card. It is used by cellular networks to identify and authenticate subscribers when they connect to mobile services. An IMSI typically consists of three parts: the Mobile Country Code (MCC), which identifies the subscriber’s country; the Mobile Network Code (MNC), which identifies the carrier; and the Mobile Subscriber Identification Number (MSIN), which uniquely identifies the subscriber within that network.From a security and privacy perspective, the IMSI is considered sensitive information because it can be used to track or profile users if exposed. To reduce this risk, modern mobile networks avoid transmitting the IMSI in plaintext over the air whenever possible, instead using temporary identifiers such as the Temporary Mobile Subscriber Identity (TMSI). Nevertheless, IMSI exposure remains a concern in scenarios involving rogue base stations (often called IMSI catchers), signaling attacks, or poorly secured network configurations.