Help Viewer in Apple Mac OS X before 10.6.2 does not use an HTTPS connection to retrieve Apple Help content from a web site, which allows man-in-the-middle attackers to send a crafted help:runscript link, and thereby execute arbitrary code, via a spoofed response.
http://www.vupen.com/english/advisories/2009/3184
http://www.securityfocus.com/bid/36956
http://support.apple.com/kb/HT3937
http://lists.apple.com/archives/security-announce/2009/Nov/msg00000.html