Today Apple released AirPort firmware 7.7.9 for their Wireless-AC base stations and 7.6.9 for their older Wireless-N base stations. As expected, these firmware updates include a fix for the high profile KRACK security problem that was discovered to affect all WiFi devices.
Though the attack has never been seen in the wild, in laboratory conditions it would allow a hacker to intercept data transmitted by WiFi, in the same way someone could if they were on the same network as you. Data that is properly secured with SSL was not vulnerable.
I recommend installing the update as soon as you are able if you are using Apple AirPort base stations. To update you will need to open the AirPort Utility in the Applications/Utilities folder on your Mac, or by using AirPort Utility on your iPhone or iPad. Click or tap each of your routers, log in if necessary (the password may or may not be the same as your WiFi password), and you should find an “update” button.