By Munsif Vengattil, Arjun Panchadar and Paresh Dave

(Reuters) - Facebook Inc (FB.O) said on Friday that hackers stole digital login codes allowing them to take over nearly 50 million user accounts in its worst security breach ever given the unprecedented level of potential access, adding to what has been a difficult year for the company's reputation.

Facebook, which has more than 2.2 billion monthly users, said it has yet to determine whether the attacker misused any accounts or stole private information. It also has not identified the attacker’s location or whether specific victims were targeted. Its initial review suggests the attack was broad in nature.

Chief Executive Mark Zuckerberg described the incident as “really serious" in a conference call with reporters. His account was affected along with that of Chief Operating Officer Sheryl Sandberg, a spokeswoman said.

Shares in Facebook fell 2.6 percent on Friday, weighing on major Wall Street stock indexes.

Facebook made headlines earlier this year after profile details from 87 million users was improperly accessed by political data firm Cambridge Analytica. The disclosure has prompted government inquiries into the company's privacy practices across the world, and fueled a "#deleteFacebook" social movement among consumers.

U.S. lawmakers said on Friday that the hack may boost calls for data privacy legislation.

“This is another sobering indicator that Congress needs to step up and take action to protect the privacy and security of social media users," Democratic U.S. Senator Mark Warner said in a statement.

Federal Trade Commission Commissioner Rohit Chopra on Twitter said "I want answers" with a link to a Reuters story on the breach.

'COMPLEX' FLAW

Facebook's latest vulnerability had existed since July 2017, but the company first identified it on Tuesday after spotting a "fairly large" increase in use of its "view as" privacy feature on Sept. 16, executives said.

"View as" allows users to verify their privacy settings by seeing what their own profile looks like to someone else. The flaw inadvertently gave the devices of "view as" users the wrong digital code, which, like a browser cookie, keeps users signed in to a service across multiple visits.

That code could allow the person using "view as" to post and browse from someone else's Facebook account, potentially exposing private messages, photos and posts. The attacker also could have gained full access to victims' accounts on any third-party app or website where they had logged in with Facebook credentials.

“The implications of this are huge," Justin Fier, director of cyber intelligence at security company Darktrace, told Reuters.