Quora has revealed that last week it discovered hackers had broken into its systems and have stolen the data of around 100 million users.
Although the exact details of the breach are not yet known, the hackers could have gained access to usernames, encrypted passwords, email addresses and imported information like contacts from other social networking sites.
There are also concerns that the private actions of users on the popular question and answer site have also been taken, including requests for answers, direct messages, and downvotes.
In a blog post, Quora CEO Adam D'Angelo stated: "We recently discovered that some user data was compromised as a result of unauthorized access to one of our systems by a malicious third party. We are working rapidly to investigate the situation further and take the appropriate steps to prevent such incidents in the future."
He also asserted that "the overwhelming majority of the content accessed was already public on Quora", but acknowledged that "the compromise of account and other private information is serious".
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Quora in the process of "notifying users whose data has been compromised", and has taken the precautionary measure of logging out all users who they suspect have been affected, with their passwords being invalidated.
This incident is the latest in a spate of large scale data breaches; last week 500 million Marriott hotel customers had their data stolen, while thousands of credit card numbers were stolen from British Airways in September.
Quora was co-founded by former Facebook employees D'Angelo and Charlie Cheever, and became available to the public in 2010. Questions are asked, answered, edited, and organized by its community of users, who are notorious for giving in-depth, and often well-researched responses to these questions.