According to PC World, Symantec's Threat Analysis team, DeepSight, has identified a flaw in version 3.0.10 of Samba that ships with OSX. It is apparently open to "Request Mulitple Heap-Based Buffer Overflow" vulnerability which affects all versions of Samba prior to v3.0.25.
Symantec noted that Apple last updated Samba as part of its security update 2005-003.
A patch was released by the samba team earlier this month but initially only Windows systems were thought to be vulnerable to this exploit.
Symantec recommends all Mac users that require Windows Sharing update their Macs with the latest version of Samba. If that is not possible, the security vendor recommends disabling Samba completely.