How did alpha bank find out about the change of the SIM card?

M

may-cat2014-11-26 20:40:30

Information Security

may-cat, 2014-11-26 20:40:30

Today, when trying to pay through alpha-click, I received a message from him.
For security reasons, the transaction cannot be confirmed. A sign of changing the SIM card has been detected. Please contact the Call Center or Alfa-Bank branch to update the SIM-card data for the Alfa-Click service.
HOW did they know that I changed my SIM card?! How is this technically possible?!

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neolink, 2014-11-26
@may-cat

With the help of an HLR request for your number, since each card has a unique IMSI, they saw that you are already online from another card
https://ru.wikipedia.org/wiki/GSM_core_network#.D0...