In the latest attack that shows how hard it is for users to identify phone numbers with premium call charges, a researcher has found that he could have earned millions by abusing the online phone verification systems used by Google, Microsoft, and Instagram.
Many websites and mobile apps allow users to associate a phone number with their account. This can be used for two-factor authentication or as an account recovery and verification option. Many of these systems rely on codes sent via text messages, but also offer the option to call the user and dictate such codes.
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