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Re: VoIP-Phones: Weakness in proccessing SIP-Notify-Messages

List,
I just wanted to inform you that I wrote a tiny script, which enables you to test the discussed vulnerability with your equipment.
This script works perfect with Grandstream BT100, Cisco phones won't work on the fly. Please refer to the README for more information. 

You can download the script here:
http://www.tele-consulting.com/index.php?where=download&part=download&what=snf
(german website, but you should be able to download it. If not, feel free to send me an eMail). 


Cheers,

Tobias Glemser
Tele-Consulting security | networking | training GmbH

Tobias Glemser wrote on 06.07.2005 18:20:

                  Tele-Consulting GmbH
            security | networking | training
advisory 05/07/06 

URL of this advisory:
http://pentest.tele-consulting.com/advisories/05_07_06_voip-phones.txt


Topic:
    Weakness in implemenation of proccessing SIP-Notify-Messages
    in VoIP-Phones.

Summary:
    Due to ignoring the value of 'Call-ID' and even 'tag' and
    'branch' while processing NOTIFY messages, VoIP-Hardphones
    process spoofed status messages like "Messages-Waiting".
According to RFC 3265, Chap 3.2 every NOTIFY has to be em- 
    bedded in a subcription mechanism. If there ain't knowledge
    of a subscription, the UAC has to respond with a "481
    Subscription does not exist" message.

    All tested phones processed the "Messages-Waiting" messages
    without prior subscriptions anywhere.

Effect:
    An attacker could send "Messages-Waiting: yes" messages to
    all phones in a SIP-environment. Almost every phone proccesses
    this status message and shows the user an icon or a blinking
    display to indicate that new messages are available on the
    voice box.
If the attacker sends this message to many recipients in a 
    huge environment, it would lead to server peaks as many users
    will call the voice box at the same time.
    Because there are no new voice messages as indicated by the
    phone the users will call the support to fix this alleged server
    problem.

    All tested phones process the message with a resetted Call-ID,
    'branch' and 'tag' sent by a spoofed IP-Adress.

Example:
    Attacker spoofs the SIP-Proxys IP, here: 10.1.1.1
    Victim 10.1.1.2
UDP-Message from Attacker to Victim Session Initiation Protocol 
         Request-Line: NOTIFY sip:login@xxxxxxxx SIP/2.0
         Message Header
             Via: SIP/2.0/UDP 15.1.1.12:5060;branch=000000000000000
             From: "asterisk" <sip:asterisk@xxxxxxxx>;tag=000000000
             To: <sip:login@xxxxxxxx>
              Contact: <sip:asterisk@xxxxxxxx>
              Call-ID: 00000000000000@xxxxxxxx
             CSeq: 102 NOTIFY
                 User-Agent: Asterisk PBX
              Event: message-summary
              Content-Type: application/simple-message-summary
              Content-Length: 37
         Message body
              Messages-Waiting: yes\n
              Voicemail: 3/2\n

Solution:
    Phones who receive a NOTIFY message to which no subscription
    exists, must send a "481 Subscription does not exist" response.
    It should be possible to use the REGISTER request as a
    non-SUBSCRIBE mechanism to set up a valid subscription.

    This would reduce the possibility of an attack in a way, that
    only with a sniffed and spoofed subcription such an attack would
    be possible. Background is given by the way dialogs are des-
    cribed in RFC 3261 and the sections 5.5 and 3.2 of RFC 3265.


Affected products:
    Cisco 7940/7960
    Grandstream BT 100
    others will be tested in future


--
Tobias Glemser


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