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.
Official Statement from Red Hat (05/23/2007)
Not vulnerable. OpenSSH supplied with Red Hat Enterprise Linux 2.1, 3, 4, and 5 does not contain S/KEY support.
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20070421 OpenSSH - System Account Enumeration if S/Key is used
20070424 OpenSSH - System Account Enumeration if S/Key is used