The remote vulnerability affects both the iPhone and Android Instagram app, and due the lack of control on the logic applied to authorization feature, it allows attachers to effect a brute force attack in the context of user application.
He demonstrated the effectiveness of the attack by adding himself to the list of users Facebook's CEO Mark Zuckerber follows through Instagram and by sending him a personal message:
Guerrero revealed details about the flaw on his personal blog (via Google Translate), and has notified Instagram about it.
The company reacted by patching the flaw in a matter of hours and by commenting that never in the course of the bug existing was users' data at risk and that at no point were private photos made public.
"The technical researcher was not able to follow private users, nor were private users' data ever at risk," they wrote. "We don't have any evidence that this bug was taken advantage of at any other scale than very minimal experiments by a technical researcher."