BlastDoor works by parsing all the data contained in an iMessage in a secure sandbox, isolated from the rest of the operating system. By doing so, if any malicious content exists inside the iMessage, it will not impact the OS.
This security mechanism takes a look at all incoming messages and inspects their content in a secure environment, which prevents any malicious code inside of a message from interacting with iOS or accessing user data.
iMessage has been the target of multiple attacks in the past, as several researchers pointed out that Apple’s messaging service was “doing a poor job of sanitizing incoming user data.” These exploits allowed hackers to take control over an iPhone by just sending a text message or photo to the device.
BlastDoor sandbox has been designed to thwart most exploits including brute force or exploitation of the shared cache on the iOS.