[babesh]

I speculate that GameKit is basically abandonware by Apple. They even got rid of the app a few years ago.

There probably hasn't been hardening of it in years and the initial work was probably developed in haste.

This is systemic. Apple has a bad habit of abandoning software that isn't a priority. So, one shouldn't be surprised that Apple hasn't fixed these exploits. And I wonder if the author has fully mined GameKit for exploits yet. Perhaps there are more to be found.

The architecture of iOS and OSX isn't conducive to security AFAIK. It is more of an add-on as one can see instead of being architected in.

[illusionofchaos]

I haven't checked further, maybe authentication token can be used to gain access to Apple account and more data. Also one other method could used to write arbitrary data outside of an app sandbox, that might be useful for further exploitation.