iCloudHack: DropBox the More Likely Source
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Another assumption is that it was the celebrity's account and only that account that was compromised in every case. It could have been JLaw's (ex-)boyfriend's iCloud account that was hacked, surely?
Some of the leaks do look like iCloud backups though. Videos don't get uploaded to Apple's "photo stream" so it would have to be an iCloud backup or, less likely, a local backup that was uploaded to Dropbox. Right now it "feels like" a lot of different leaks and leakers jumping on 4chan and demanding bitcoins for their own little dumps.
If not, then perhaps there's a way to test if the files came from Dropbox or not.