Evidently, someone has discovered a Unicode bug in iOS, where if an app receives a specific Unicode sequence (infinitely-repeating Unicode code point), the OS will kill the Springboard as it allocates too much memory, restarting it immediately. I can't include the sequence here, because SE detects it and denies including it in the body.
Now, this doesn't seem like a serious issue; "Oh, my phone was reset randomly. Great.", but this is posing a great risk to the availability of iOS devices everywhere. For instance, if I know a person that has an iOS device, I could send this message
anonymously through a fake Apple account via iMessage on a Mac OSX/iOS device to reset their phone (even in the middle of a call, as this is a notification bug).
If I were truly malicious, I could write a script on Mac OSX to generate a list of (semi)-random phone numbers to send this message to every X minutes, effectively permanently putting their device in a resetting state (preventing them from patching, doing anything important, etc).
Apple is aware of the issue and their devs are fixing it, but a huge amount of users delay updates as long as possible, or simply don't bother to update; this means that the issue could be a permanent threat to them.
Methods I've thought of to fix the issue that seem impossible:
1.) Push a hot-fix to iOS devices (is this even implemented?)
2.) Force an update to the latest version of the OS (seems to breach privacy/ToS agreement)
Since this poses a threat to the availability of Apple devices, I figured I would ask the security experts: how can Apple ensure that this issue does not plague (software-outdated) iOS devices in the coming years?
Edit: The problem is two-fold:
1.) Pre-send validation in iOS requires an update, as the code to whitelist Unicode characters obviously doesn't exist yet, otherwise this wouldn't be an issue; this is why I asked the question.
2.) This problem can't realistically be fixed server-side as Apple is relaying the message, because Apple encrypts the messages before they are sent, and if they decrypted the messages to strip out bad Unicode, it would be a breach of users' privacy.