Facebook Inc. revealed a software bug recently exposed millions of photos, including pictures users had not posted.

This incident, which occurred from Sept. 13 to Sept. 25, may have affected up to 6.8 million users and up to 1,500 apps built by 876 developers, the company said in a Dec. 14 blog post. The company added that it has fixed the bug.

The photo API bug affected users who gave third-party apps approval to access their photos. App developers may have been exposed to photos that people began uploading to the site but did not post publicly, including those in Facebook Stories or Facebook's Marketplace.

Facebook said it will roll out tools for app developers to determine which users were impacted by the software bug. Users who were potentially impacted by this breach will be notified via an alert on Facebook.

Earlier this year, the social media site estimated that up to 87 million users may have been affected due to a breach by Cambridge Analytica LLC.