OpenSSH-portable (OpenSSH) 3.6.1p1 and earlier with PAM support enabled immediately sends an error message when a user does not exist, which allows remote attackers to determine valid usernames via a timing attack.
http://marc.info/?l=bugtraq&m=105172058404810&w=2
http://marc.info/?l=bugtraq&m=106018677302607&w=2
https://cert-portal.siemens.com/productcert/pdf/ssa-412672.pdf