Facebook on Friday apologized to millions of users who may have had their unpublished photos exposed to third-party app developers.

Axar.az reports citing Euronews.

"We're sorry this happened," Tomer Bar, engineering director at Facebook, wrote in a blog post about the flaw.

The flaw allowed apps that users accessed through the social network's "Facebook Login" system to see photos that had been uploaded but not published on Facebook, as well as photos published to Facebook's "Marketplace" and to its Stories feature.

Facebook directed users concerned that their private photos may have been exposed to visit its help page, which has a tool for users to determine if they were affected.