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
draft-ietf-tls-ssl2-must-not-04.txt
Abstract
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
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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.
Turner & Polk Expires June 16, 2011 [Page 1]
Internet-Draft Prohibiting SSL 2.0 December 2010
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",
"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:
o Message authentication uses MD5 [MD5]. Most security-aware users
have already moved away from any use of MD5
[I-D.turner-md5-seccon-update].
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.
Turner and Polk Expires June 16, 2011 [Page 2]
Internet-Draft Prohibiting SSL 2.0 December 2010
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.
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
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. IANA Considerations
None.
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.
Turner and Polk Expires June 16, 2011 [Page 3]
Internet-Draft Prohibiting SSL 2.0 December 2010
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-
progress.
Turner and Polk Expires June 16, 2011 [Page 4]
Internet-Draft Prohibiting SSL 2.0 December 2010
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.gov
Turner and Polk Expires June 16, 2011 [Page 5]
