Network Working Group                                         S. Turner
Internet Draft                                                     IECA
Updates: 5246 (once approved)                                  Tim Polk
Intended Status: Standards Track                                   NIST
Expires: April 8, 2011                                  October 8, 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) Protocol, RFC

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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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
   ( 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) [TLS] for security services.  This is a good thing, but some
   TLS clients and servers also support negotiating the use of SSL
   version 2.0 [SSL2]; however, this version does not provide the
   expected 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.

   This document updates the backward compatibility sections found in
   the Transport Security Layer (TLS) Protocol [TLS] and earlier

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.

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

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    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 negotiate or use SSL 2.0.

    o TLS clients MUST NOT send SSL 2.0 CLIENT-HELLO messages.

    o TLS servers MUST NOT negotiate or use SSL 2.0.

   As described in [TLS], TLS servers that do not support SSL 2.0 MAY
   accept version 2.0 CLIENT-HELLO messages as the first message of a
   TLS handshake for interoperability with old clients.

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.
   We would also like to thank Michael D'Errico, Paul Hoffman, Nikos
   Mavrogiannopoulos, Yngve Pettersen, Marsh Ray, Martin Rex, and Yaron
   Sheffer 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.

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

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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-

 Authors' Addresses

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


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


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