WebKit in Apple Safari before 5.1.4 does not properly implement "From third parties and advertisers" cookie blocking, which makes it easier for remote web servers to track users via a cookie.
http://www.securitytracker.com/id?1026785
http://secunia.com/advisories/48377
http://lists.apple.com/archives/security-announce/2012/Mar/msg00003.html