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.
The good (and either timely, or coincidental) news is, that the same developers have confirmed this exploit has just been patched. For now, however, the code lives on, only now marked as a "proof of concept." We've reached out to Apple for comment, but until there's any official word either way, this is one feasible possibility. There are of course a number of other potential routes into user accounts (not least the good old fashioned abuse of trust of a close colleague or friend, or romantic interest). What's unusual here, is the apparent scale of the issue, with numerous celebrities suffering leaks all at the same time.
At the time of writing, Reddit was clamping down on people naming the alleged leakers, and picture hosting site Imgur is pulling any uploads of the images as best it can, 4chan also displayed rare twitchiness, and pulled the original thread. Likewise, with Twitter reportedly suspending accounts that share the images, you might want to think twice before you RT -- it's fair to say, the internet is officially in a spin.
Update: The Next Web has contacted the author of ibrute, asking if it could have been used to obtain the leaked images. The response: "I've not seen any evidence yet, but I admit that someone could use this tool."
Update 2: Overnight, Apple has confirmed that it is conducting internal investigations. While it still isn't clear where the hacks originated from, Apple is advocating any one concerned about security to activate two-stage authentication. Details of which can be found here.
Matt Brian contributed to this report.