Quora, the popular question-and-answer website, said Monday evening that hackers broke into one of its systems and compromised information from approximately 100 million users.
CEO Adam D'Angelo said in a blog post the company discovered last week that a malicious third party had gained unauthorized access to one of its systems.
Account information, including names, email addresses and encrypted passwords, may have been illegally accessed, according to the post. User-imported data from other social networks could also have been taken.
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But the majority of the content the hackers accessed was already public on the site — such as questions, answers, and comments, D'Angelo wrote.
"We're in the process of notifying users whose data has been compromised," he said.
Quora is investigating the issue and said it has "notified law enforcement officials."
Massive data breaches have become a common occurrence as hackers exploit vulnerabilities in the information technology systems used by businesses.
Last week, Marriott International said hackers had copied data from the reservation database for its Starwood Hotels brand, with unauthorized access dating back to 2014.
Earlier in the year, social networking giant Facebook disclosed a security breach where 29 million users had their personal information exposed.
This story first appeared on CNBC.com. More from CNBC: