IAX2 in Asterisk Open Source 1.2.x before 1.2.31, 1.4.x before 1.4.23-rc4, and 1.6.x before 1.6.0.3-rc2; Business Edition A.x.x, B.x.x before B.2.5.7, C.1.x.x before C.1.10.4, and C.2.x.x before C.2.1.2.1; and s800i 1.2.x before 1.3.0 responds differently to a failed login attempt depending on whether the user account exists, which allows remote attackers to enumerate valid usernames.
http://www.vupen.com/english/advisories/2009/0063
http://www.securitytracker.com/id?1021549
http://www.securityfocus.com/bid/33174
http://www.securityfocus.com/archive/1/499884/100/0/threaded
http://www.debian.org/security/2009/dsa-1952
http://securityreason.com/securityalert/4910
http://security.gentoo.org/glsa/glsa-200905-01.xml
http://secunia.com/advisories/37677
http://secunia.com/advisories/34982