What exploit? From the article:

"... according to people familiar with the matter."

There is absolutely no proof of an exploit here. There are a lot of people 'familiar with the matter' that talk BS. For example I heard some expert say that they just bought 1000 iPhones and copied the ROM on each phone to try each pin combination.

Edit: to be clear: I'm talking about a software exploit. I think it's more likely the phone was cracked via a hardware exploit.