WhatsApp's mass adoption stems in part from how easy it is to find a new contact on the messaging platform: Add someone's phone number, and WhatsApp instantly shows whether they're on the service, and often their profile picture and name, too.

Repeat that same trick a few billion times with every possible phone number, it turns out, and the same feature can also serve as a convenient way to obtain the cell number of virtually every WhatsApp user on earth—along with, in many cases, profile photos and text that identifies each of those users. The result is a sprawling exposure of personal information for a significant fraction of the world population.

One group of Austrian researchers have now shown that they were able to use that simple method of checking every possible number in Whats

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