In the rush to get a fix out for a potentially dangerous vulnerability in macOS High Sierra, Apple broke file sharing. Fortunately, though, the company is aware of the issue and has already supplied a fix. The fix is a workaround and a new patch is not needed.
From support doc HT208317:
If you experience issues with authenticating or connecting to file shares on your Mac after you install Security Update 2017-001 for macOS High Sierra 10.13.1, follow these steps to repair file sharing:
- Open the Terminal app, which is in the Utilities folder of your Applications folder.
- Type sudo /usr/libexec/configureLocalKDC and press Return.
- Enter your administrator password and press Return.
- Quit the Terminal app.
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