Safari RSS in Apple Mac OS X 10.5.8 and 10.6.x before 10.6.5 does not block Java applets in an RSS feed, which allows remote attackers to obtain sensitive information via a feed: URL containing an applet that performs DOM modifications.
http://support.apple.com/kb/HT4435
http://lists.apple.com/archives/security-announce/2010//Nov/msg00000.html