Facebook has apologized for a photo API bug that may have exposed photos belonging to 6.8 million users.

This issue affects people who use Facebook login and gave third-party apps permission to access their photos. Normally, access is limited to images posted on users' timelines. In this case, the bug expanded access to include photos shared on Marketplace or Facebook Stories. Third-party apps may have had access to a larger set of photos from Sep. 13 to Sep. 25, 2018.

The bug also affected photos that people uploaded to Facebook but chose not to post yet. When someone uploads a photo to Facebook but the post doesn't complete (because they lost service, for example), Facebook stores a copy for three days so they can finish the post later.

Facebook says the only apps affected by the bug are those it had previously approved to access the photos API, and which people had authorized to access their images. That said, the flaw may have affected up to 1,500 apps built by 876 developers, the company says in a blog post.

The social network is notifying potentially affected users with a Facebook alert, which will direct them to the Help Center where they can see if they've used affected apps. It suggests users log into apps with which they've shared Facebook photos, and check which images they shared.

Read more details here.