Apple has today released firmware updates for Time Capsules and AirPort Base Stations which address the vulnerabilities known as KRACK.
Firmware update 7.6.9 is available for AirPort Express, AirPort Extreme, and Time Capsules with 802.11n; 7.7.9 is for AirPort Extreme and Time Capsules with 802.11ac.
All users with AirPort Base Stations or Time Capsules should download and install these as soon as possible, using AirPort Utility for macOS or iOS. If you cannot obtain the update, check that you are running the current version of the utility. The macOS version is installed with macOS updates, but for iOS it is available in the iOS App Store.