Apple has just released a Security Update for Snow Leopard that apparently fixes an issue where remote hackers could access your AFP shared folders without a password. This appears to be the only fix included in the update.
Apple describes the problem and the fix:
“A remote attacker with knowledge of an account name on a target system may bypass the password validation and access AFP shared folders. By default, File Sharing is not enabled. This issue does not affect systems prior to Mac OS X v10.6.”
The Security Update 2010-006 is recommended for all users and can be found via Software Update or as a direct download here.
[Apple via Software Update]