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Internet Engineering Task Force (IETF) S. Turner Request for Comments: 6176 IECA Updates: 2246, 4346, 5246 T. Polk Category: Standards Track NIST ISSN: 2070-1721 March 2011

         Prohibiting Secure Sockets Layer (SSL) Version 2.0


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

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 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.

Turner & Polk Standards Track [Page 1] RFC 6176 Prohibiting SSL 2.0 March 2011

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 that TLS clients and servers never
 negotiate the use of SSL version 2.0.
 RFC 4346 [TLS1.1], and later RFC 5246 [TLS1.2], 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 the following:
 o  Message authentication uses MD5 [MD5].  Most security-aware users
    have already moved away from any use of MD5 [RFC6151].
 o  Handshake messages are not protected.  This permits a man-in-the-
    middle to trick the client into picking a weaker cipher suite than
    it 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
 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.

Turner & Polk Standards Track [Page 2] RFC 6176 Prohibiting SSL 2.0 March 2011

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 ClientHello
    message that 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 ClientHello messages in the
    version 2 CLIENT-HELLO format as specified in RFC 5246 [TLS1.2],
    Appendix E.2.  Note that this does not contradict the prohibition
    against actually negotiating the use of SSL 2.0.
 o  TLS servers MUST NOT reply with an SSL 2.0 SERVER-HELLO with a
    protocol version that 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. Security Considerations

 This entire document is about security considerations.

5. Acknowledgements

 The idea for this document was inspired by discussions between Peter
 Saint Andre, Simon Josefsson, and others on the Extensible Messaging
 and Presence Protocol (XMPP) mailing list.
 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.

Turner & Polk Standards Track [Page 3] RFC 6176 Prohibiting SSL 2.0 March 2011

6. References

6.1. 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.

6.2. 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.
 [RFC6151]   Turner, S. and L. Chen, "Updated Security Considerations
             for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
             RFC 6151, March 2011.

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

Turner & Polk Standards Track [Page 4]

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