There's a publicly available jailbreak for fully updated iPhones.
An iPhone jailbreaker using the handle "Pwn20wnd" released a jailbreak yesterday (Aug. 18) that works on most iPhones, iPads and iPod Touches running iOS 12.4, and all iDevices running iOS 12.2 or earlier. Several people on Twitter said the procedure works fine.
The jailbreak, called unc0ver, does not work on iOS 12.3 at all, and devices with A12 chipsets (the iPhone XS, XS Max and XR or the 2019 iPad Mini and iPad Air) need to be running iOS 12.2 or earlier.
"If you are not already jailbroken-> Upgrade or downgrade to iOS 12.4 ASAP," Pwn20wnd wrote on Twitter. "If you are not already jailbroken on A12-A12X with iOS 12.3.x/13.0 -> Upgrade or downgrade to iOS 12.4 ASAP. In any other case -> Just stay wherever you are." (iOS 13 is available as a beta test release.)
Jailbreaks allow iDevice users to make system modifications and install third-party apps not authorized by Apple. The downside is that they seriously compromise iOS security, but they're not illegal to use in the United States.
Instructions for the jailbreak are at https://github.com/pwn20wndstuff/Undecimus/blob/master/README.md. Tom's Guide takes no responsibility for damage or any other type of injury or loss caused by installing this jailbreak.
The risks of jailbreaking are real. One iOS security expert told VICE Motherboard that malicious websites could embed this jailbreak code into a web page, which could then be used to jailbreak an iDevice without the user's knowledge. He also said Apple made this jailbreak possible by unfixing a flaw that had been patched in iOS 12.3, but didn't elaborate further.
Another iOS expert, Stefan Esser, tweeted that "I hope people are aware that with a public jailbreak being available for the latest iOS 12.4 people must be very careful what Apps they download from the Apple AppStore. Any such app could have a copy of the jailbreak in it."
It's very likely that Apple will have an update to kill the jailbreak within a day or two. The jailbreak does not seem to survive a device reboot, which makes it not worth much to spies or criminals. But iOS jailbreaks that survive reboots and can be installed remotely can sell for millions of dollars.