CloudKit data, and most iCloud data outside of iCloud Drive, is sandboxed to individual applications. This makes sense from the standpoint of securing user data from leaking from one application to another without their control. However, with my own iCloud credentials as a user, I have access to all of my data via the apps which own the individual buckets.
Is it possible, as a technical user writing code on my own machine (not something that would be distributed in the App Stores), to enumerate, read, and/or write data as myself in the iCloud buckets of applications which I did not create?
I am particularly interested to do this on a Mac (with developer tools and unsigned apps allowed), and am willing to assume that I know the bundle IDs of the buckets of interest. Being able to enumerate all buckets which exist for my user would be even more useful.
If you have the same developer account as the original app, then you could create a 2nd app that could use the same CloudKit container. You do have to be aware that there is a developer and a production database. You can only access the production database with a published app (Could be a TestFlight only distribution)
There is a way to access a container that is created by someone else. But then you do need to get an API access web token which can only be handed out by the developer account of the original app. You could then access the container using the CloudKit Web API