Yahoo uncovered breach after probing a black market sale

A hacker’s attempt to sell user data he claimed was stolen from Yahoo actually led the company to uncover a far more severe breach.

Yahoo confirmed Thursday a data breach, which affects at least 500 million users, but it could be unrelated to the black market sale of alleged Yahoo accounts, according to a source familiar with the matter.

The information comes even as security experts have been questioning why Yahoo took so long to warn the public when it was known that a hacker was claiming to be selling the data online around early August.

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