It doesn't even have to be an undisclosed vulnerability. iOS 9.3 was released today and included several fixes to vulnerabilities which allowed arbitrary code

It doesn't even have to be an undisclosed vulnerability. iOS 9.3 was released today and included several fixes to vulnerabilities which allowed arbitrary code execution, including one with "kernel privileges" [1] Now that these vulns are out in the open, maybe someone has offered to help the FBI use them to gain access to the phone. [1] https://support.apple.com/en-us/HT206166 https://support.apple.com/en-us/HT206166