File sharing broken after installing Security Update 2017-001
Apple has already issued a quick fix to this. Follow these steps:


Launch the Terminal app (usually found in the Utilities folder within your Applications folder)
Copy the following command and paste it into Terminal:


sudo /usr/libexec/configureLocalKDC


Press enter
Enter your Admin password (Note: You won’t actually see the cursor move as you enter your password)
Press enter


For more info: https://support.apple.com/en-us/HT208317