About 50 million user accounts were hacked. The stock declined 3.4% to $163 on Friday after the news was released.
"On the afternoon of Tuesday, September 25, our engineering team discovered a security issue affecting almost 50 million accounts," Facebook's vice president of product management, Guy Rosen, said in the post Friday. "We're taking this incredibly seriously and wanted to let everyone know what's happened and the immediate action we've taken to protect people's security."
Hackers exposed a coding vulnerability on the platform, allowing them to access tokens, or keys to user accounts. The hackers unlocked the accounts and took them over. Facebook said it has fixed the vulnerability and has reset the tokens of the roughly 50 million users affected.
Facebook said it's still investigating and doesn't yet know who the attackers were and whether or not they misused users information. "We're working hard to better understand these details - and we will update this post when we have more information, or if the facts change."
CEO Mark Zuckerberg said "This is a very serious security issue, and we're taking it very seriously," according to CNBC.
This is the second attack on the platform of this magnitude Facebook has experienced this year, the first one being the Cambridge Analytica scandal. The stock, which got hit hard by both the Cambridge Analytica scandal, and a disappointing earnings and guidance report in July, has fallen 10.3% this year.