In a blog post titled "Quora Security Update", the site revealed that the data of approximately 100 million users may have been compromised as a result of unauthorised access to its systems on Friday.
Quora, the question-and-answer site that bludgeons users into signing up, has been hit with a major data breach.
That's because to access a Quora page, you have to log in - either by creating an new account or by linking your Google or Facebook account.
All Quora users have been logged out and affected users will be forced to change their passwords when they next log in.
Users' account information, including names, email addresses, encrypted passwords and data imported from linked networks when authorised by users may have been compromised, he said.
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"It is our responsibility to make sure things like this don't happen, and we failed to meet that responsibility", Quora CEO Adam D'Angelo wrote in Monday's post.
The company said it is logging out all Quora users who may have been affected to prevent further damage.
"We have retained a leading digital forensics and security firm to assist us", it said.
"Questions and answers that were written anonymously are not affected by this breach as we do not store the identities of people who post anonymous content", he added. One possibility is that the hackers - should they be able to decipher the stolen encrypted passwords - will try to use the passwords on other websites that a person logs on to, possibly including sites that contain more sensitive information such as credit card details.
It also said that the root cause has been identified, with steps being taken to address the issue. Users who chose the same password to protect accounts on a different service should immediately reset those passwords as well.