Twitter user axi0mX revealed the bootrom exploit, dubbed checkm8, that makes it possible to jailbreak the devices. As The Verge explains, bootrom exploits take advantage of a vulnerability in the initial code that iOS devices load when they boot up. Because it's read-only memory (ROM), Apple can't overwrite or patch it through a software update. In other words, it's permanent.
Fortunately, the exploit requires physical access to a phone and has to be carried out over USB, so while hundreds of millions of iPhones are at risk, it's unlikely that many of them will be impacted. As far as we know, there's no jailbreak using the checkm8 exploit yet. But there is a risk that the exploit could be developed and not require physical access in the future, which could leave millions of devices vulnerable. We've reached out to Apple for comment on this and will update the story if we hear anything.