[…] the unknown attackers were able to achieve an unprecedented level of access by exploiting a vulnerability in an undocumented hardware feature that few if anyone outside of Apple and chip suppliers such as ARM Holdings knew of.
Other questions remain unanswered, wrote Larin, even after about 12 months of intensive investigation. Besides how the attackers learned of the hardware feature, the researchers still don’t know what, precisely, its purpose is.
My conspiratorial side actually went the route that the chip supplier placed it without Apple’s knowledge/consent. Obviously nothing to support that, but [if it was intentional] I think it’s more likely than Apple signing off on it.
Wow people are so quick to jump to conspiracy theories. Special undocumented functions are very common in complex chips, being there to test the chip. I would go so far as to say any complex chip will have special registers/special pins which are used for testing. Testing can be a significant portion of a chip’s cost, so anything which can reduce testing time is highly desirable from the chip maker’s point of view. The article even mentioned that they believed the special registers were there for testing.