The good news is that if you’ve upgraded to the latest iOS 9.3.1 release then you’re already protected because the security researchers who discovered the vulnerability – Patrick Kelley and Matt Harrigan – didn’t publish details until Apple patched the hole.
Of course, the bad news is that there are many, many people who haven’t yet upgraded – and several who never will because their iOS devices are too old.
Exploiting the bug is devastatingly simple. iOS devices automatically look around the Wi-Fi neighborhood for known hotspots. If they find one, they it join automatically. The example quoted by Krebs is attwifi, which is the public Wi-Fi offered in North American outlets of Starbucks. Once a customer joins this, an iPhone will automatically connect to it whenever the customer is in ANY Starbucks outlet.
As soon as the iOS device gets online via Wi-Fi, it attempts to ensure the system time is set correctly by pinging a network time protocol (NTP) server. You might’ve already worked out what’s coming next: by creating a fake attwifi Wi-Fi hotspot, and installing a fake NTP server on it, the researchers made the iOS device set its own date to 1 Jan 1970 – at which point it began the perennial reboot cycle that it’s impossible to break out of, short of waiting for the battery to run down, just like with the original 1970 bug.
Even worse, one of the test iPads used by the researchers entered a weird countdown condition, with the date going even further back in time, wherein it also got incredibly hot.
What can you do to protect yourself? Update to 9.3.1 if you haven’t already (open Settings, tap General and then Software Update). Other than that, be careful which public Wi-Fi hotspots you join. You might consider activating the Ask To Join Networks option in Settings > Wi-Fi, in order to stop iOS automatically joining “known” networks.