If you are running macOS High Sierra 10.13.1 and have just applied the Security Update (2017-001) to fix its root user vulnerability, you may find that you cannot access file sharing on that Mac any more. Problems include being unable to authenticate or connect to that Mac to access its shared files.

Apple has issued a note recommending a solution which it says should repair file sharing in this circumstance. On the Mac which is supposed to be sharing its files (as a ‘server’), enter the following in Terminal’s command line:
sudo /usr/libexec/configureLocalKDC
then enter your admin user’s password at the prompt.

This command generates a local Kerberos Key Distribution Centre (LocalKDC) for Open Directory single sign-on and sets it up, using the System keychain and other installed components.

You should also be able to check whether the LocalKDC is functioning correctly by entering the command
sudo /usr/libexec/checkLocalKDC
and authenticating. That should return Success; if it doesn’t, then you should need to run configureLocalKDC as above.

Running configureLocalKDC is not destructive, and if repeated when it is not needed shouldn’t cause any problems.

(Thanks to Miles Wolbe for alerting me to this.)