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Date: Thu, 18 Feb 2021 11:37:09 -0600
From: "Asterisk Security Team" <>
Subject: [FD] AST-2021-002: Remote crash possible when negotiating T.38

               Asterisk Project Security Advisory - AST-2021-002

          Product         Asterisk                                            
          Summary         Remote crash possible when negotiating T.38         
     Nature of Advisory   Denial of service                                   
       Susceptibility     Remote authenticated sessions                       
          Severity        Minor                                               
       Exploits Known     No                                                  
        Reported On       December 8, 2020                                    
        Reported By       Gregory Massel                                      
         Posted On        
      Last Updated On     February 5, 2021                                    
      Advisory Contact    kharwell AT sangoma DOT com                         
          CVE Name        CVE-2021-26717                                      

      Description     When re-negotiating for T.38 if the initial remote      
                      response was delayed just enough Asterisk would send    
                      both audio and T.38 in the SDP. If this happened, and   
                      the remote responded with a declined T.38 stream then   
                      Asterisk would crash.                                   
    Modules Affected  res_pjsip_session.c, res_pjsip_t38.c                    

    Resolution  When re-negotiating for T.38, and a delay occurs Asterisk     
                now sends SDP only for the expected T.38 stream. A check was  
                also put in place to ensure an active T.38 media stream is    
                active within Asterisk when attempting to change state for    

                               Affected Versions
                Product               Release Series  Introduced              
          Asterisk Open Source             16.x       16.15.0                 
          Asterisk Open Source             17.x       17.9.0                  
          Asterisk Open Source             18.x       18.1.0                  
           Certified Asterisk              16.8       16.8-cert4              

                                  Corrected In
                 Product                              Release                 
           Asterisk Open Source               16.16.1, 17.9.2, 18.2.1         
            Certified Asterisk                       16.8-cert6               

                               Patch URL                               Revision   Asterisk   
                                                                      16      Asterisk   
                                                                      17      Asterisk   
                                                                      18    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         
    February 1, 2021     Kevin Harwell           Initial revision             

               Asterisk Project Security Advisory - AST-2021-002
               Copyright �� 2021 Digium, Inc. All Rights Reserved.
  Permission is hereby granted to distribute and publish this advisory in its
                           original, unaltered form.

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