Apple iTunes before 10.5.1 does not properly verify the authenticity of updates, which allows man-in-the-middle attackers to execute arbitrary code via a Trojan horse update, as demonstrated by evilgrade and DNS cache poisoning.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A17136
http://www.infobyte.com.ar/down/isr-evilgrade-1.0.0.tar.gz
http://www.infobyte.com.ar/down/Francisco%20Amato%20-%20evilgrade%20-%20ENG.pdf
http://support.apple.com/kb/HT5030
http://lists.apple.com/archives/Security-announce/2011/Nov/msg00003.html
http://archives.neohapsis.com/archives/bugtraq/2008-07/0250.html