If guest access is not your wish, you must take actioin to limit access to shared items.
1) To eliminate anonymous access:
Deactivate "Guest Access"
2) To grant access to specific individuals:
Use the "User and Group" Functions
3) In all cases, when designating shared items:
Immediately configure permissions for "owner," "group," and "everyone."
Please note: An "item" is at minimum a folder and at maximum a volume (which may consist of an entire hard drive).