How you could steal money from Instagram, Microsoft and Google with help from a premium rate phone number
Researcher Arne Swinnen found an ingenious way to make money from the likes of Google, Microsoft and Instagram – getting their two-factor authentication registration schemes to call a premium rate phone number:
"They all offer services to supply users with a token via a computer-voiced phone call, but neglected to properly verify whether supplied phone numbers were legitimate, non-premium numbers. This allowed a dedicated attacker to steal thousands of EUR/USD/GBP/… Microsoft was exceptionally vulnerable to mass exploitation by supporting virtually unlimited concurrent calls to one premium number"
Swinnen told the tech companies concerned about the issue. Despite the fact that it was clear that no customer data was being put at risk through the technique (the actual potential damage was for the tech companies to lose some cash), the researcher was awarded $2000 and $500 by Instagram’s and Microsoft’s respective bug bounties.
You can learn more in Arne Swinnen’s blog post.
A warning. If you use a call based multi-factor authentication you check that you users doesn’t use a premium number. If not, it might cost you a lot of money.