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@10.1.1.2 SIP/2.0
       Message Header
            Via: SIP/2.0/UDP 15.1.1.12:5060;branch=000000000000000
            From: "asterisk" <sip:asterisk@10.1.1.1>;tag=000000000
            To: <sip:login@10.1.1.2>
             Contact: <sip:asterisk@10.1.1.1>
             Call-ID: 00000000000000@10.1.1.1
            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


TTTTTTT CCCC
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