Their "Blastdoor" in iOS 14 is an effort to address this (by sandboxing more parts of the message processing pipeline):With the number of exploits coming in through iMessage vulnerabilities, it is reminiscent of Flash. Apple may need to refactor the iMessage code base with security in mind from the ground up. I am not sure specifically what Apple is doing from a development side on iOS 15, but it needs work.
A Look at iMessage in iOS 14
Posted By Samuel Groß, Project Zero On December 20, Citizenlab published “ The Great iPwn ”, detailing how “Journalists [were] Hacked ...
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Apparently there's still some work to do ...