What is a Rapid Security Response for iPhone?

How do I install an Apple Rapid Security Response? What does iOS Rapid Security Response do?

Apple Rapid Security Response

Apple has released Rapid Security Responses for iOS 16.4.1 and later to quickly fix urgent security problems without going through a full operating system update. This is the first time these rapid response patches have been released.

Apple plans to use Rapid Security Response to deliver security patches on the iPhone, iPad, and Mac more efficiently when needed. These patches are designed to address specific security problems and do not include feature changes. All Apple devices are set to automatically install new Rapid Security Responses, and users will be prompted to restart their device to apply the patch if necessary.

To check this setting on your iPhone, navigate to Settings -> General -> Software Update -> Automatic Updates and toggle Security Responses & System Files -> ON (green switch).