libsecurity in Apple Mac OS X before 10.7.2 does not properly handle errors during processing of a nonstandard extension in a Certificate Revocation list (CRL), which allows remote attackers to execute arbitrary code or cause a denial of service (application crash) a crafted (1) web site or (2) e-mail message.
http://www.securityfocus.com/bid/50085
http://support.apple.com/kb/HT5002
http://lists.apple.com/archives/Security-announce/2011//Oct/msg00003.html