The FTP server in Apple Mac OS X 10.4.8 and earlier, when FTP Access is enabled, will crash when a login failure occurs with a valid user name, which allows remote attackers to cause a denial of service (crash) and enumerate valid usernames.
https://exchange.xforce.ibmcloud.com/vulnerabilities/30621
http://www.vupen.com/english/advisories/2006/4750
http://www.us-cert.gov/cas/techalerts/TA06-333A.html
http://www.securityfocus.com/bid/21335
http://www.kb.cert.org/vuls/id/371648
http://securitytracker.com/id?1017303
http://secunia.com/advisories/23155
http://lists.apple.com/archives/security-announce/2006/Nov/msg00001.html