6176
PROPOSED STANDARD
Prohibiting Secure Sockets Layer (SSL) Version 2.0
Authors: S. Turner, T. Polk
Date: March 2011
Area: sec
Working Group: tls
Stream: IETF
Abstract
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). [STANDARDS-TRACK]
RFC 6176
PROPOSED STANDARD
Updated by: 8996 Errata Exist
Internet Engineering Task Force (IETF) S. Turner
Request for Comments: 6176 IECA
Updates: <a href="./rfc2246">2246</a>, <a href="./rfc4346">4346</a>, <a href="./rfc5246">5246</a> T. Polk
Category: Standards Track NIST
ISSN: 2070-1721 March 2011
<span class="h1">Prohibiting Secure Sockets Layer (SSL) Version 2.0</span>
Abstract
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 <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="https://www.rfc-editor.org/info/rfc6176">http://www.rfc-editor.org/info/rfc6176</a>.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
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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.
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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
Many protocols specified in the IETF rely on Transport Layer Security
(TLS) [<a href="#ref-TLS1.0" title=""The TLS Protocol Version 1.0"">TLS1.0</a>][TLS1.1][<a href="#ref-TLS1.2" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">TLS1.2</a>] 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 [<a href="#ref-SSL2" title=""The SSL Protocol"">SSL2</a>]; 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.
<a href="./rfc4346">RFC 4346</a> [<a href="#ref-TLS1.1" title=""The Transport Layer Security (TLS) Protocol Version 1.1"">TLS1.1</a>], and later <a href="./rfc5246">RFC 5246</a> [<a href="#ref-TLS1.2" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">TLS1.2</a>], 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 [<a href="#ref-TLS1.0" title=""The TLS Protocol Version 1.0"">TLS1.0</a>][TLS1.1][<a href="#ref-TLS1.2" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">TLS1.2</a>].
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Requirements Terminology</span>
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
[<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. SSL 2.0 Deficiencies</span>
SSL version 2.0 [<a href="#ref-SSL2" title=""The SSL Protocol"">SSL2</a>] deficiencies include the following:
o Message authentication uses MD5 [<a href="#ref-MD5" title=""The MD5 Message-Digest Algorithm"">MD5</a>]. Most security-aware users
have already moved away from any use of MD5 [<a href="./rfc6151" title=""Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms"">RFC6151</a>].
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.
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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Changes to TLS</span>
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 <a href="./rfc5246">RFC 5246</a> [<a href="#ref-TLS1.2" title=""The Transport Layer Security (TLS) Protocol Version 1.2"">TLS1.2</a>],
<a href="#appendix-E.2">Appendix E.2</a>. 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.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Security Considerations</span>
This entire document is about security considerations.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Acknowledgements</span>
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.
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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. References</span>
<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-TLS1.0">TLS1.0</a>] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
<a href="./rfc2246">RFC 2246</a>, January 1999.
[<a id="ref-TLS1.1">TLS1.1</a>] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", <a href="./rfc4346">RFC 4346</a>, April 2006.
[<a id="ref-TLS1.2">TLS1.2</a>] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", <a href="./rfc5246">RFC 5246</a>, August 2008.
<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. Informative References</span>
[<a id="ref-MD5">MD5</a>] Rivest, R., "The MD5 Message-Digest Algorithm", <a href="./rfc1321">RFC 1321</a>,
April 1992.
[<a id="ref-SSL2">SSL2</a>] Hickman, Kipp, "The SSL Protocol", Netscape
Communications Corp., Feb 9, 1995.
[<a id="ref-RFC6151">RFC6151</a>] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
<a href="./rfc6151">RFC 6151</a>, March 2011.
Authors' Addresses
Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Fairfax, VA 22031
USA
EMail: [email protected]
Tim Polk
National Institute of Standards and Technology
100 Bureau Drive, Mail Stop 8930
Gaithersburg, MD 20899-8930
USA
EMail: [email protected]
Turner & Polk Standards Track [Page 4]
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