Apple has a permanent security update page located at https://support.apple.com/en-us/HT201222

Recent updates of note include: an update to Safari for OS X 10.11 though 10.13, Security Update 2018-001 for macOS 10.13, and iOS 11.3.1 for iPhones and iPads.

While Apple has a strong reputation for robust security, users are strongly encouraged to install all security updates as soon as they become available, for every device they have that connects to the Internet. In addition to Macs and iOS devices, this includes Apple TVs, HomePods, Apple Watches -- if it connects to the Internet, update it.