The login module in Sun Java System Access Manager 6 2005Q1 (aka 6.3), 7 2005Q4 (aka 7.0), and 7.1 responds differently to a failed login attempt depending on whether the user account exists, which allows remote attackers to enumerate valid usernames.
https://exchange.xforce.ibmcloud.com/vulnerabilities/48283
http://www.vupen.com/english/advisories/2009/0269
http://www.securityfocus.com/bid/33489
http://sunsolve.sun.com/search/document.do?assetkey=1-66-242026-1
http://sunsolve.sun.com/search/document.do?assetkey=1-21-119465-15-1