Behavioral discrepancy information leak in Juniper Netscreen VPN running ScreenOS 5.2.0 and earlier, when using IKE with pre-shared key authentication, allows remote attackers to enumerate valid usernames via an IKE Aggressive Mode packet, which generates a response if the username is valid but does not respond when the username is invalid.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2005-2641
http://www.securityfocus.com/bid/14595
http://www.nta-monitor.com/news/vpn-flaws/juniper/netscreen/index.htm
http://securitytracker.com/id?1014728