Account information, including name, email address, encrypted password and data imported from linked networks when authorized by users may have been compromised, it said. It also includes public content and actions like users' questions, answers, comments, upvotes, etc and non-public content and actions - such as answer requests, downvotes, direct messages.
Anonymously authored questions and answers were reportedly not affected by the breach, and the company said it doesn't store information from anonymous users.
"There's little hope of sharing and growing the world's knowledge if those doing so can not feel safe and secure, and can not trust that their information will remain private", the Quora founder wrote.
It further notes, "On Friday we discovered that some user data was compromised by a third party who gained unauthorized access to one of our systems".
On the security threat, Quora said, "We believe we've identified the root cause and taken steps to address the issue, although our investigation is ongoing and we'll continue to make security improvements".
Quora relies on the website traffic that comes from people searching questions in their browsers. Users affected by the breach should receive an email with information.
The service has logged out all affected users, and in the event they use passwords to authenticate, old passwords have been invalidated.
If you want to simply delete your Quora account, visit your account privacy settings and choose "Delete Account". Anyone who had reused a password elsewhere is advised to create a new password for that account too. Law enforcement has been notified, it added. "We have already taken steps to ensure the situation is contained, and we are working to prevent this type of event from happening in the future".