The design of the Internet Key Exchange (IKE) protocol, when using Aggressive Mode for shared secret authentication, does not encrypt initiator or responder identities during negotiation, which may allow remote attackers to determine valid usernames by (1) monitoring responses before the password is supplied or (2) sniffing, as originally reported for FireWall-1 SecuRemote.
https://exchange.xforce.ibmcloud.com/vulnerabilities/10034
http://www.securityfocus.com/bid/5607
http://www.nta-monitor.com/news/checkpoint.htm
http://www.kb.cert.org/vuls/id/886601
http://www.checkpoint.com/techsupport/alerts/ike.html
http://marc.info/?l=bugtraq&m=103176164729351&w=2
http://marc.info/?l=bugtraq&m=103124812629621&w=2
http://lists.grok.org.uk/pipermail/full-disclosure/2002-September/001223.html