It hasn't been a good week for Apple's Mac software team. First, anwas discovered that gave anyone access to a computer running MacOS High Sierra simply by signing in with the username "Root" and no password.
To its credit, after the bug was shared on Twitter, Apple users began to report that it broke file sharing. Ugh.. Then, after installing the update,
Thankfully, there's already a fix. According to The Loop, Apple reissued the update with a fix for file sharing. So if you installed the security update yesterday, open the App Store app on your Mac and check for updates again. The security update will show up again as if you never installed it. Install it and you should be good to go.
If you'd rather be safe, you can manually fix the bug yourself according to an Apple Support document. You'll need to open the Terminal app on your Mac (Applications > Utilities > Terminal) and type or paste the following command:
Press Return, enter your administrator password, and press Return again. You can then close Terminal and test file sharing if you were having issues.