How do you define proof? There really isn't even any proof that iCloud was compromised. What is known is that most of the photos had metadata showing the pictures were taken from Apple devices and some websites have people talking about exploiting iCloud. The only real proof is Apple's press release saying that certain accounts were targeted.
If metadata and website boasting is your standard of proof, then
this link should suffice. The author scoured various forums and concluded that iCloud was the most popular target but other platforms were hacked as well. This makes sense as a vast majority of celebrities probably use iPhones.
Here is
another link that states that many pictures and videos could not have been from iCloud. The author writes,
Dropbox or Google Drive
Despite the original leaker claiming to have accessed the trove of photos thanks to an iCloud exploit, the range of devices showcased suggests that another service may have been to blame. Various naked celebrities are photographed taking selfies with Android devices and webcams. Leaked videos could not have originated from the iCloud photo backup service. The range of devices and media may mean that another backup service like Dropbox or Google Drive could be the originator of the leaked photos, with both services offering automatic backup tools for photos and videos imported from cellphones.
Snapchat
Several of the leaked celebrity photos had text overlaid, which indicates that at least some of the photographs were first sent through Snapchat. While Snapchat has struggled with security issues in the past, it's unlikely that the app was the source of all the nude photos. Rather, it may be that it was either accessed as part of a larger hack, or screenshots of images received through the app were discovered after hacking into a backup service.
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Here is
an interview were someone states that Google's cloud was also to blame.
JUDY WOODRUFF: And staying with you, Sean Gallagher, so what questions does this raise about the so-called cloud? And, by the way, remind everybody what the cloud is. It’s not actually a cloud. What is it?
SEAN GALLAGHER: Right.
Well, the cloud is computers in a data center attached to the Internet. In this case, they were computers at a data center owned by Apple. Also, there was data stolen from devices that were on Amazon — pardon me — on Google’s cloud.
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Celebrity accounts on many different platforms have been hacked over the years. The only thing that makes this instance special is that so many names were involved. Each celebrity probably has a different story for how their images got in the hands of these creeps. Sadly, though, instead of finding out those stories the narrative has devolved into "its all Apple's fault."