Prohibiting Secure Sockets Layer (SSL) Version 2.0
draft-ietf-tls-ssl2-must-not-04

Network Working Group                                         S. Turner 
Internet Draft                                                     IECA 
Updates: 5246, 4346, 2246 (once approved)                       T. Polk 
Intended Status: Standards Track                                   NIST 
Expires: June 16, 2011                                December 16, 2010 
 
                                      
                        Prohibiting SSL Version 2.0 
                    draft-ietf-tls-ssl2-must-not-04.txt 

Abstract 

   This document requires that when TLS clients and servers establish 
   connections that they never negotiate the use of Secure Sockets Layer 
   (SSL) version 2.0.  This document updates the backward compatibility 
   sections found in the Transport Security Layer (TLS). 

Status of this Memo 

   This Internet-Draft is submitted in full conformance with the 
   provisions of BCP 78 and BCP 79.  This document may contain material 
   from IETF Documents or IETF Contributions published or made publicly 
   available before November 10, 2008. 

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   This Internet-Draft will expire on May 16, 2009. 

Copyright Notice 

   Copyright (c) 2010 IETF Trust and the persons identified as the 
   document authors. All rights reserved. 

 
 
 
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   This document is subject to BCP 78 and the IETF Trust's Legal 
   Provisions Relating to IETF Documents 
   (http://trustee.ietf.org/license-info) in effect on the date of 
   publication of this document. Please review these documents 
   carefully, as they describe your rights and restrictions with respect 
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   include Simplified BSD License text as described in Section 4.e of 
   the Trust Legal Provisions and are provided without warranty as 
   described in the Simplified BSD License. 

1. Introduction 

   Many protocols specified in the IETF rely on Transport Layer Security 
   (TLS) [TLS1.0][TLS1.1][TLS1.2] for security services.  This is a good 
   thing, but some TLS clients and servers also support negotiating the 
   use of Secure Sockets Layer (SSL) version 2.0 [SSL2]; however, this 
   version does not provide a sufficiently high level of security. SSL 
   version 2.0 has known deficiencies. This document describes those 
   deficiencies, and it requires TLS clients and servers never negotiate 
   the use of SSL version 2.0. 

   TLS 1.1 [RFC4346] and later in TLS 1.2 [RFC5246] explicitly warned 
   implementers that the "ability to send version 2.0 CLIENT-HELLO 
   messages will be phased out with all due haste."  This document 
   accomplishes this by updating the backward compatibility sections 
   found in TLS [TLS1.0][TLS1.1][TLS1.2]. 

1.1. Requirements Terminology 

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 
   "OPTIONAL" in this document are to be interpreted as described in 
   [RFC2119]. 

2. SSL 2.0 Deficiencies 

   SSL version 2.0 [SSL2] deficiencies include: 

   o Message authentication uses MD5 [MD5].  Most security-aware users 
     have already moved away from any use of MD5 
     [I-D.turner-md5-seccon-update]. 

   o Handshake messages are not protected.  This permits a man-in-the-
     middle to trick the client into picking a weaker cipher suite than 
     they would normally choose. 

 
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   o Message integrity and message encryption use the same key, which is 
     a problem if the client and server negotiate a weak encryption 
     algorithm. 

   o Sessions can be easily terminated.  A man-in-the-middle can easily 
     insert a TCP FIN to close the session and the peer is unable to 
     determine whether or not it was a legitimate end of the session. 

3. Changes to TLS 

   Because of the deficiencies noted in the previous section: 

   o TLS clients MUST NOT send the SSL version 2.0 compatible CLIENT-
     HELLO message format. Clients MUST NOT send any client hello 
     message which specifies a protocol version less than 
     { 0x03, 0x00 }. As previously stated by the definitions of all 
     previous versions of TLS, the client SHOULD specify the highest 
     protocol version it supports.  

   o TLS servers MAY continue to accept CLIENT-HELLO messages in the 
     version 2 CLIENT-HELLO format as specified in TLS 1.2 [RFC5246] 
     Appendix E.2. Note that this does not contradict the prohibition 
     against actually negotiating the use of SSL 2.0.  

     TLS Servers MUST NOT reply with a SSL 2.0 SERVER-HELLO with a 
     protocol version which is less than { 0x03, 0x00 } and instead 
     MUST abort the connection, i.e., when the highest protocol version 
     offered by the client is { 0x02, 0x00 } the TLS connection will be 
     refused.  

   Note that the number of servers that support this above-mentioned 
   "MAY accept" implementation option is declining, and the SSL 2.0 
   CLIENT-HELLO precludes the use of TLS protocol enhancements that 
   require TLS extensions. TLS extensions can only be sent as part of an 
   (Extended) ClientHello handshake message. 

4. IANA Considerations 

   None. 

5. Security Considerations 

   This entire document is about security considerations. 

6. Acknowledgements 

   The idea for this document was inspired by discussions between Peter 
   Saint Andre, Simon Josefsson, and others on the XMPP mailing list.  
 
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   We would also like to thank Michael D'Errico, Paul Hoffman, Nikos 
   Mavrogiannopoulos, Tom Petch, Yngve Pettersen, Marsh Ray, Martin Rex, 
   Yaron Sheffer, and Glen Zorn for their reviews and comments. 

7. References 

7.1. Normative References 

   [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate 
                    Requirement Levels", BCP 14, RFC 2119, March 1997. 

   [TLS1.0]         Dierks, T., and C. Allen, "The TLS Protocol Version 
                    1.0", RFC 2246, January 1999. 

   [TLS1.1]         Dierks, T. and E. Rescorla, "The Transport Layer 
                    Security (TLS) Protocol Version 1.1", RFC 4346, 
                    April 2006. 

   [TLS1.2]         Dierks, T. and E. Rescorla, "The Transport Layer 
                    Security (TLS) Protocol Version 1.2", RFC 5246, 
                    August 2008. 

7.2. Informative References 

   [MD5]            Rivest, R., "The MD5 Message-Digest Algorithm", RFC 
                    1321, April 1992.  

   [SSL2]           Hickman, Kipp, "The SSL Protocol", Netscape 
                    Communications Corp., Feb 9, 1995. 

   [I-D.turner-md5-seccon-update] Turner, S., and L. Chen, "Updated 
                    Security Considerations for the MD5 Message-Digest 
                    Algorithm", draft-turner-md5-seccon-update, work-in-
                    progress. 

 
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 Authors' Addresses 

   Sean Turner 
   IECA, Inc. 
   3057 Nutley Street, Suite 106 
   Fairfax, VA 22031 
   USA 

   EMail: turners@ieca.com 

   Tim Polk 
   National Institute of Standards and Technology 
   100 Bureau Drive, Mail Stop 8930 
   Gaithersburg, MD 20899-8930 
   USA 

   EMail: tim.polk@nist.gov  

    

 
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