"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," Quora CEO Adam D'Angelo wrote in a blog post on Monday night.
Quora discovered the breach on November 30. It found that that some user data was compromised by a third party who gained unauthorised access to one of its systems.
"We're still investigating the precise causes and in addition to the work being conducted by our internal security teams, we have retained a leading digital forensics and security firm to assist us. We have also notified law enforcement officials," D'Angelo said.
"We are working rapidly to investigate the situation further and take the appropriate steps to prevent such incidents in the future.
"We're very sorry for any concern or inconvenience this may cause," he added.
Quora was founded by D'Angelo, a former Chief Technology Officer at Facebook, in 2009.
The Mountain View, California-headquartered company has over 300 million monthly unique visitors.
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