File sharing poses security risks, especially if you make an entire hard drive available for sharing by creating Administrator-level accounts in Mac OS X. When you make an entire hard disk available for sharing, you either have to trust everyone you let access it or you have to go to a lot of trouble. You have to set specific access privileges for various folders that you want to keep network users out of, and changing the ownership or group for a folder must be done at the UNIX command-line. There is always the risk of forgetting to deny access to a folder that you don't want network users to share.

Guests used to pose a particular security risk under Mac OS 9 file sharing because guests could connect without a password. Now, because guest users have access only to Public folders, this security risk is substantially lessened, because Public folders are read-only. However, if the guest has write access, such as to the Drop Box, he or she can still cause mischief by filling your disk.

An administrator's special ability to connect to all disks and work virtually without restrictions also threatens the computer's security. Anyone who can learn or guess the administrator's owner name and password can crack your computer from another computer on a network.

And, because all file sharing is over IP, all these security risks are amplified by the fact that your potential hacker does not have to be physically near your Mac — the hacker could enter by modem or Internet connection.

Here are some techniques for improving file-sharing security:

• Be very sure to turn off Administrator access on any account that does not absolutely require it.
• Turn off write access for guests, even to Drop Box folders.
• Do not overvalue the security of registered users and passwords. An authorized person may connect to one of your shared folders from any computer on the network and then leave his or her computer without disconnecting. Someone else can then come along and use this computer to access all your shared files (subject to the access privileges you set). Remind people who connect to your shared folders that they must put away all your shared folders (by ejecting them) when they are finished. Also, remind users to lock their keychains and/or log out of the Mac. If they don't, unauthorized users may be able to access shared folders using their account, even if they do not know the password.
• Particularly if you have high-speed Internet access, such as a cable modem, DSL, or ISDN, invest in a firewall. Firewalls provide a barrier to unauthorized access from outside your local network. Most routers, such as those from Asante and LinkSys, come with firewalls.