The User Documentation component in Apple Mac OS X through 10.6.8 uses http sessions for updates to App Store help information, which allows man-in-the-middle attackers to execute arbitrary code by spoofing the http server.
http://www.securityfocus.com/bid/50085
http://support.apple.com/kb/HT5002
http://lists.apple.com/archives/Security-announce/2011//Oct/msg00003.html