Facebook blamed this new leak on a Photo API bug that was present in its backend code between September 13 to September 25, 2018.
The company said that during that interval the bug allowed Facebook third-party apps to access more than just the user’s public photos. Tomer Bar, a Facebook developer, provided the following explanation about the Photo API bug leak:
When someone gives permission for an app to access their photos on Facebook, we usually only grant 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 – we store a copy of that photo so the person has it when they come back to the app to complete their post.
Bar said that a Facebook investigation revealed that 1,500 apps built by 876 developers might have been able to access the non-public photos of up to 6.8 million users.
Earlier this year, a data scientist working for Cambridge Analytica revealed the company had several years ago used the system to gather data on tens of millions of Americans.