Java for Mac OS X 10.5 before Update 6 and 10.6 before Update 1 accepts expired certificates for applets, which makes it easier for remote attackers to execute arbitrary code via an applet.
http://www.securityfocus.com/bid/37206
http://support.apple.com/kb/HT3970
http://support.apple.com/kb/HT3969
http://secunia.com/advisories/37581
http://lists.apple.com/archives/security-announce/2009/Dec/msg00001.html
http://lists.apple.com/archives/security-announce/2009/Dec/msg00000.html