Facebook said it had fixed a bug that may have exposed private photos of up to 6.8 million users.

The bug allowed some 1,500 applications to access private photos for 12 days ending Sept. 25, Facebook said.

“We’re sorry this happened,” it said in a blog.

Facebook said it discovered a photo API bug that may have affected people who used Facebook Login and granted permission to third-party apps to access their photos. Because of this bug, some third-party apps may have had access to a broader set of photos than usual for 12 days.

When someone gives permission for an app to access their photos on Facebook, Facebook usually only grants the app access to photos people share on their timeline. In this case, the bug potentially gave developers access to other photos, such as those shared on Marketplace or Facebook Stories. The bug also impacted photos that people uploaded to Facebook but chose not to post. For example, if someone uploads a photo to Facebook but doesn't finish posting it - maybe because they've lost reception or walked into a meeting - Facebook stores a copy of that photo so the person has it when they come back to the app to complete their post.

Early next week Facebook promised to roll out tools for app developers that will allow them to determine which people using their app might be impacted by this bug.

Facebook will also notify the people potentially impacted by this bug via an alert on Facebook. The notification will direct them to a Help Center link where they'll be able to see if they've used any apps that were affected by the bug.

Facebook is also recommending people log into any apps with which they have shared their Facebook photos to check which photos they have access to.

The incident prompted privacy regulators in Europe to start investigations of Facebook. The Irish Data Protection Commissioner (DPC), the lead regulator of Facebook in the European Union, said on Friday that it was investigating to determine whether the company had complied with strict new EU privacy rules in its response to a number of breaches, including the latest one that exposed photos.

Facebook said in a statement that it was in close contact with the Irish regulator and happy to answer any questions.

The European data law requires companies to report data breaches to authorities within 72 hours, giving regulators authority to impose fines of up to 4 percent of annual global revenue for infractions.