SSH protocol 2 (aka SSH-2) public key authentication in the development snapshot of OpenSSH 2.3.1, available from 2001-01-18 through 2001-02-08, does not perform a challenge-response step to ensure that the client has the proper private key, which allows remote attackers to bypass authentication as other users by supplying a public key from that user's authorized_keys file.
https://exchange.xforce.ibmcloud.com/vulnerabilities/6084
http://www.openbsd.org/advisories/ssh_bypass.txt
http://online.securityfocus.com/bid/2356
http://archives.neohapsis.com/archives/bugtraq/2001-02/0159.html