Yahoo has discovered a 3-year-old security breach that enabled a hacker to compromise more than 1 billion user accounts, breaking the company's own humiliating record for the biggest security breach in history.

The digital heist disclosed on Wednesday occurred in August 2013, more than a year before a separate hack that Yahoo announced nearly three months ago. That breach affected at least 500 million users, which had been the most far-reaching hack until the latest revelation.

"It's shocking," said security expert Avivah Litan of Gartner Inc.