An exploit has been uncovered by security researchers that would allow basically any iPhone, from the 2011 iPhone 4s to the 2017 iPhone X and 8/Plus, to be permanently jailbroken. It is related to the Apple A chips inside these devices, and apparently, from the Apple A5 all the way to the Apple A11, they are all vulnerable.
The bootrom exploit is dubbed checkm8 (checkmate) and is apparently not only permanent, but also unpatcheable. For now, physical access is required to apply the exploit in a part of the read-only memory that Apple can’t write on via a patch, hence making it permanent. It is unknown whether a follow-up version would evolve in a way that would not require physical access to the device, in which case millions of iPhones would be affected.