OpenSSH 4.6 and earlier, when ChallengeResponseAuthentication is enabled, allows remote attackers to determine the existence of user accounts by attempting to authenticate via S/KEY, which displays a different response if the user account exists, a similar issue to CVE-2001-1483.
http://lists.grok.org.uk/pipermail/full-disclosure/2007-April/053906.html
http://lists.grok.org.uk/pipermail/full-disclosure/2007-April/053951.html
http://securityreason.com/securityalert/2631