Asterisk Open Source 1.2.x before 1.2.32, 1.4.x before 1.4.24.1, and 1.6.0.x before 1.6.0.8; Asterisk Business Edition A.x.x, B.x.x before B.2.5.8, C.1.x.x before C.1.10.5, and C.2.x.x before C.2.3.3; s800i 1.3.x before 1.3.0.2; and Trixbox PBX 2.6.1, when Digest authentication and authalwaysreject are enabled, generates different responses depending on whether a SIP username is valid, which allows remote attackers to enumerate valid usernames.
https://exchange.xforce.ibmcloud.com/vulnerabilities/45059
http://www.vupen.com/english/advisories/2009/0933
http://www.securityfocus.com/bid/34353
http://www.debian.org/security/2009/dsa-1952
http://security.gentoo.org/glsa/glsa-200905-01.xml
http://secunia.com/advisories/37677
http://secunia.com/advisories/34982
http://downloads.asterisk.org/pub/security/AST-2009-003.html