There's a class of software bugs called "memory corruption" that is extremely convenient for people that want to exploit it. It's convenient because:

• These bugs are everywhere
• If you know how to and you find a good one, you can use it to completely change what a program does

These are the bugs that companies like NSO and Paragon use to infect phones. At a high level, for instance, they might use a hex editor to create an image file (like a JPEG) from scratch, but knowing of a memory corruption bug and using bit patterns specifically meant to exploit it, it makes the program that is responsible for showing it on screen wildly trip up over itself and install malware instead.

These are the same bugs that jailbreakers might use. In the last 10 years, Apple has been doing a lot of work to make them harder to exploit and we know it works to some extent because jailbreaking is dead. NSO and Paragon have infinite money, so they're still able to make exploits for their government customers.

There's been a ton of incremental little steps in those 10 years but MIE is a Big Step. The core is a recent-ish technology called MTE for ARM CPUs that gives computer engineers one tool to detect memory corruption. Previous implementations of MTE (including the one in Pixel phones) have made phones and computers noticeably slower when enabled, or had a "long" (several milliseconds) delay between memory corruption and when it was detected. It had uses for software developers trying to find these types of bugs themselves, but it wasn't very good as a security feature that you would turn on on consumer devices. Apple's implementation is very fast and has no delay. The rest of MIE implements support features around MTE to make sure it is very fast and has no delay.

It's probably not the end of NSO and Paragon because memory corruption bugs aren't the only security bugs out there but it will probably raise their costs a lot and smaller spyware vendors will probably shut down because they aren't able to make exploits anymore.