The potential exploit relates to a project on the code hosting site Github called, imaginatively, ibrute. Just a day before the images leaked, the developers of ibrute announced a bug in the Find My iPhone service means it doesn't employ bruteforce protection (i.e. an attack can continue using different passwords until the right one if found). The implication is that this could give access to AppleIDs, and from there any number of avenues to compromise accounts become significantly more viable. It's certainly not the first intrusion issue with the service we've seen. If this was the flaw used, the hackers would have needed email addresses of celebrities. But, it's possible that only one address is needed, allowing to search inboxes for those of others in a domino effect.