Sometimes when I have spare time I click around to see which Mac users forgot to disable Guest sharing.

Unfortunately this sometimes turns out to be quite a number of people (see fig: GuestEnabled).

Some even allowing read and write access to certain directories which you probably don't want (e.g. ~/).

In the Finder sidebar we see this nice list of other Macs and PCs on the network (see fig: Network).

This can be used to easily find a networked computer to initiate file or screen sharing.

When Guest sharing is enabled this allows other to login as a Guest to access some shared files (see fig: GuestAccess).

Guest sharing is ENABLED by default on new Macs.

By default this only allows guests to access the Public folder in your home folder (~/).

This Public folder is read and write for you and read only for everyone else.

However, it contains a folder called 'Drop Box' that is write only for everyone else.

This means that guests can 'drop' stuff into this Drop Box.

As seen in the screenshot above (fig: GuestAccess), that particular Mac only has 29 GB of free space left.

If you are on a network with evil people (e.g. a conference with people from a competing experiment),

they could simply try to fill your dropbox with 29 GB of random stuff, like so:

Which would mean your hard drive would fill up, causing problems with virtual memory, saving of files, crashes, etc.

And it would probably take you some time to figure out the cause.

Luckily, that is very simple!

(see fig: UsersGroups)

And you're done..

You can also check the File Sharing settings in the Sharing PreferencePane,

to see if you were perhaps sharing other folders.

Note; Disabling Guest sharing does not prevent you from access file sharing using your username & password,

as long as you leave file sharing enabled.