Facebook just announced a major security issue affecting 50 million users

Mark Zuckerberg

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Facebook CEO Mark Zuckerberg.

Facebook just disclosed a major security issue affecting almost 50 million accounts.

The flaw, discovered on Tuesday, allowed attackers to take over Facebook users' accounts, and the company's stock dropped 3% on the news.Advertisement

"Access tokens are the equivalent of digital keys that keep people logged in to Facebook so they don't need to re-enter their password every time they use the app."

Facebook says it's not yet clear who is behind the attack.
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It was disclosed in a blog post published on Friday. The company is holding a press call on Friday morning with the media; Business Insider will attend and update this story with more information as it becomes available.

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