Asterisk Open Source 1.2.x before 1.2.35, 1.4.x before 1.4.26.3, 1.6.0.x
before 1.6.0.17, and 1.6.1.x before 1.6.1.9; Business Edition A.x.x, B.x.x
before B.2.5.12, C.2.x.x before C.2.4.5, and C.3.x.x before C.3.2.2;
AsteriskNOW 1.5; and s800i 1.3.x before 1.3.0.5 generate different error
messages depending on whether a SIP username is valid, which allows remote
attackers to enumerate valid usernames via multiple crafted REGISTER
messages with inconsistent usernames in the URI in the To header and the
Digest in the Authorization header.