There are options in macOS to automatically install various types of updates, labeled as 'app updates', 'macOS updates', 'system data files and security updates'. Maybe m6g6a has the last one checked.

I opened App Store and just had it waiting for me. Where the name would normally be in the Updates tab, it just reads "Install this update as soon as possible." [0]

[0] https://imgur.com/a/bMUKO

Edit: According to Apple's statement, they will automatically start installing the update on systems running 10.3.1 later today.

Whoever installed the OS agreed to it. http://images.apple.com/legal/sla/docs/macOS1013.pdf:

”Q. Automatic Updates. The Apple Software will periodically check with Apple for updates to the Apple Software. If an update is available, the update may automatically download and install onto your computer and, if applicable, your peripheral devices. _By using the Apple Software, you agree that Apple may download and install automatic updates onto your computer and your peripheral devices_. You can turn off automatic updates altogether at any time by changing the automatic updates settings found within System Preferences.”

It's a massive global security vulnerability with huge amounts of public exposure (so any malicious user is well aware they can take advantage). If they did, wouldn't be surprised and I'd be glad they did.

I opened App Store, found the package waiting and hit install, waited for a few minutes until it finished. So not automatically for me.

They do have this capability, and have for years, although they rarely use it. They're force-pushing this update later today. It says so right in their statement:

"the update is available for download, and starting later today it will be automatically installed on all systems running the latest version (10.13.1) of macOS High Sierra"

The update didn't auto-install for me. I suppose it only does that when "Install system data files and security updates" is enabled in System Preferences -> App Store.

Not for me, it's waiting for me to click install in the App Store. Its pretty well highlighted though.