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Versions: (draft-turner-ssl-must-not)   00 01 02         Standards Track
          03 04 rfc6176                                                 
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


   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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   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
   to this document. Code Components extracted from this document must
   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",
   "OPTIONAL" in this document are to be interpreted as described in

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

   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

   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

   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


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-

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 Authors' Addresses

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

   EMail: turners@ieca.com

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

   EMail: tim.polk@nist.gov

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