Apple has released macOS High Sierra 10.13.2 and iOS 11.2.2 with, among other things, security fixes for the recently reported Spectre vulnerability.
macOS High Sierra 10.13.2 is a free update for all customers who have a compatible machine. The update can be downloaded using the Software Update function in the Mac App Store.
iOS 11.2.2 can be downloaded for free on all eligible devices over-the-air in the Settings app. Go to Settings --> General --> Software Update.
