Apple pushed a security update for the huge High Sierra vulnerability yesterday, introducing a bug while they were at it. You should install the update as soon as possible and then do this, if File Sharing isn’t working:
Open the Terminal app, which is in the Utilities folder of your Applications folder.
sudo /usr/libexec/configureLocalKDCand press Return.
- Enter your administrator password and press Return.
- Quit the Terminal app.
John Gruber summarized the problem, which seems to have been around for a few months now:
So the exploit was floating around, under the radar, for weeks at least, but it seems as though no widespread harm came of it.
Personally, I’d call this much too optimistic — people could have been hacked without them even realizing it.