Attack surface is made of the amount of code that is running when an attacker speaks to your machine. Imagine a freshly installed GNU/Linux distro with no services. The attack surface is minimal. All packages sent to your machine will only ever be touched by relatively limited parts of the linux TCP/IP stack and NIC driver. If you now run a web server, the package coes through the NIC driver, TCP/IP stack and web server. The surface is increased. Each of these parts of your machine’s code could have bugs. The more code your attacker’s packet runs through, the more opportunity to make your machine do things you don’t like.

If you want your machine to do what you like but not what random attackers like, it is therefore mandatory to have the least amount of attack surface, not adding code in contact with your attacker like a WAF or “antivirus”. Both these kind of softwares will inspect the packages coming in an take decisions (potentially bad ones) based on the content.

WAFs will mostly not help you since on a well configured and patched system, little known bugs are exposed. They might help you occasionally but usually patching the system is more effective. Of you want this to happen automatically, it’s entirely possible. Most os’s allow automatic unattended upgrades.