Event Monitor in Apple Mac OS X before 10.6.3 does not properly validate hostnames of SSH clients, which allows remote attackers to cause a denial of service (arbitrary client blacklisting) via a crafted DNS PTR record, related to a "plist injection issue."
http://support.apple.com/kb/HT4077
http://lists.apple.com/archives/security-announce/2010//Mar/msg00001.html