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After exploiting all the vulnerabilities, the JavaScript exploit can do whatever it wants to the device including running spyware, but the attackers chose to: (a) launch the IMAgent process and inject a payload that clears the exploitation artefacts from the device; (b) run a Safari process in invisible mode and forward it to a web page with the next stage. The web page has a script that verifies the victim and, if the checks pass, receives the next stage: the Safari exploit.
I wonder what "verifies the victim" means. Sounds like they have targeted an individual with this crazy attack chain. This wouldn't be the first case iirc.