A security breach at the question-and-answer site Quora has reportedly left the personal data of millions of users in the hands of hackers. CEO Adam D’Angelo wrote in a blog post Monday that “a malicious third party” had gained access to Quora accounts, leaving the names, email addresses, and encrypted passwords for as many as 100 million users compromised. “The overwhelming majority of the content accessed was already public on Quora, but the compromise of account and other private information is serious,” D’Angelo said. “We believe we’ve identified the root cause and [have] taken steps to address the issue, although our investigation is ongoing and we’ll continue to make security improvements,” he said. The breach could even extend to data from Facebook and Twitter if users linked the social-media sites to their Quora accounts.