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Internet Engineering Task Force (IETF)                         S. TurnerRequest for Comments: 6176                                          IECAUpdates:2246,4346,5246                                        T. PolkCategory: Standards Track                                           NISTISSN: 2070-1721                                               March 2011Prohibiting Secure Sockets Layer (SSL) Version 2.0Abstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6176.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Turner & Polk                Standards Track                    [Page 1]

RFC 6176                   Prohibiting SSL 2.0                March 20111.  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 laterRFC 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",   "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 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      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.Turner & Polk                Standards Track                    [Page 2]

RFC 6176                   Prohibiting SSL 2.0                March 20113.  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 inRFC 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 20116.  References6.1.  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   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.govTurner & Polk                Standards Track                    [Page 4]