Asterisk Project Security Advisory – IAX2 provides a different response during authentication when a user does not exist, as compared to when the password is merely wrong. This allows an attacker to scan a host to find specific users on which to concentrate password cracking attempts.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/73690/AST-2009-001.txt
Source: https://packetstormsecurity.com/files/73690/Asterisk-Project-Security-Advisory-AST-2009-001.html