Asterisk Project Security Advisory - AST-2017-005

         Product        Asterisk                                              
         Summary        Media takeover in RTP stack                           
    Nature of Advisory  Unauthorized data disclosure                          
      Susceptibility    Remote Unauthenticated Sessions                       
         Severity       Critical                                              
      Exploits Known    No                                                    
       Reported On      May 17, 2017                                          
       Reported By      Klaus-Peter Junghanns                                 
        Posted On       
     Last Updated On    August 30, 2017                                       
     Advisory Contact   Joshua Colp <jcolp AT digium DOT com>                 
         CVE Name       

    Description  The "strictrtp" option in rtp.conf enables a feature of the  
                 RTP stack that learns the source address of media for a      
                 session and drops any packets that do not originate from     
                 the expected address. This option is enabled by default in   
                 Asterisk 11 and above.                                       
                 The "nat" and "rtp_symmetric" options for chan_sip and       
                 chan_pjsip respectively enable symmetric RTP support in the  
                 RTP stack. This uses the source address of incoming media    
                 as the target address of any sent media. This option is not  
                 enabled by default but is commonly enabled to handle         
                 devices behind NAT.                                          
                 A change was made to the strict RTP support in the RTP       
                 stack to better tolerate late media when a reinvite occurs.  
                 When combined with the symmetric RTP support this            
                 introduced an avenue where media could be hijacked. Instead  
                 of only learning a new address when expected the new code    
                 allowed a new source address to be learned at all times.     
                 If a flood of RTP traffic was received the strict RTP        
                 support would allow the new address to provide media and     
                 with symmetric RTP enabled outgoing traffic would be sent    
                 to this new address, allowing the media to be hijacked.      
                 Provided the attacker continued to send traffic they would   
                 continue to receive traffic as well.                         

    Resolution  The RTP stack will now only learn a new source address if it  
                has been told to expect the address to change. The RTCP       
                support has now also been updated to drop RTCP reports that   
                are not regarding the RTP session currently in progress. The  
                strict RTP learning progress has also been improved to guard  
                against a flood of RTP packets attempting to take over the    
                media stream.                                                 

                               Affected Versions       
                         Product                       Release  
                  Asterisk Open Source                  11.x    11.4.0        
                  Asterisk Open Source                  13.x    All Releases  
                  Asterisk Open Source                  14.x    All Releases  
                   Certified Asterisk                   11.6    All Releases  
                   Certified Asterisk                   13.13   All Releases  

                                  Corrected In
          Product                              Release                        
    Asterisk Open Source               11.25.2, 13.17.1, 14.6.1               
     Certified Asterisk                11.6-cert17, 13.13-cert5               

                                SVN URL                               Revision    Asterisk  
                                                                      11      Asterisk  
                                                                      13      Asterisk  
                                                                      14    Certified 
                                                                      11.6 Certified 


    Asterisk Project Security Advisories are posted at                                                             
    This document may be superseded by later versions; if so, the latest      
    version will be posted at                                             and                        

                                Revision History
        Date        Editor                    Revisions Made                  
    May 30, 2017  Joshua Colp  Initial Revision                               

               Asterisk Project Security Advisory - AST-2017-005
              Copyright (c) 2017 Digium, Inc. All Rights Reserved.
  Permission is hereby granted to distribute and publish this advisory in its
                           original, unaltered form.

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