Owners of older iPhones have been urged to install an emergency update to keep themselves safe.
Without the new software, users are at risk of having their devices taken over and attacked by hackers.
But the download is free and relatively straightforward to install, and protects people from the dangerous security bug.
The update fixes the same problem that was made public last week, and fixed in iOS 15. But some older iPhones that are still in use – the iPhone 5S and 6 – are not running the latest software, and so did not get a version of that update.
Now Apple has released a new version of iOS 12, the most recent update for those phones, that fixes the problem.
It is the first time in a year that Apple has pushed an update for that operating system, demonstrating the danger and potential effects of the security issue.
What’s more, Apple has said that the security problem may already have been exploited by attackers, leading experts to urge that the update is installed as soon as possible.
As well as older iPhones, the update is available for some iPads and iPod touch. Many of those are still in use but may not be updated as often, since they are not able to run the latest software.
The new update can be downloaded by heading to the Settings app, clicking General and then choosing Software Update. That will check with Apple’s servers and show the option to download and install any pending update.
iOS 12 does include the option for automatic updates, and so users may find that their phone has already downloaded the latest patch.