draft-ietf-tls-ssl2-must-not-04.txt   rfc6176.txt 
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 Internet Engineering Task Force (IETF) S. Turner
draft-ietf-tls-ssl2-must-not-04.txt Request for Comments: 6176 IECA
Updates: 2246, 4346, 5246 T. Polk
Abstract Category: Standards Track NIST
ISSN: 2070-1721 March 2011
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 Prohibiting Secure Sockets Layer (SSL) Version 2.0
This Internet-Draft is submitted in full conformance with the Abstract
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 This document requires that when Transport Layer Security (TLS)
Task Force (IETF), its areas, and its working groups. Note that clients and servers establish connections, they never negotiate the
other groups may also distribute working documents as Internet- use of Secure Sockets Layer (SSL) version 2.0. This document updates
Drafts. the backward compatibility sections found in the Transport Layer
Security (TLS).
Internet-Drafts are draft documents valid for a maximum of six months Status of This Memo
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 This is an Internet Standards Track document.
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at This document is a product of the Internet Engineering Task Force
http://www.ietf.org/shadow.html. (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.
This Internet-Draft will expire on May 16, 2009. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6176.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
1. Introduction 1. Introduction
Many protocols specified in the IETF rely on Transport Layer Security 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 (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 thing, but some TLS clients and servers also support negotiating the
use of Secure Sockets Layer (SSL) version 2.0 [SSL2]; however, this use of Secure Sockets Layer (SSL) version 2.0 [SSL2]; however, this
version does not provide a sufficiently high level of security. SSL version does not provide a sufficiently high level of security. SSL
version 2.0 has known deficiencies. This document describes those version 2.0 has known deficiencies. This document describes those
deficiencies, and it requires TLS clients and servers never negotiate deficiencies, and it requires that TLS clients and servers never
the use of SSL version 2.0. negotiate the use of SSL version 2.0.
TLS 1.1 [RFC4346] and later in TLS 1.2 [RFC5246] explicitly warned RFC 4346 [TLS1.1], and later RFC 5246 [TLS1.2], explicitly warned
implementers that the "ability to send version 2.0 CLIENT-HELLO implementers that the "ability to send version 2.0 CLIENT-HELLO
messages will be phased out with all due haste." This document messages will be phased out with all due haste". This document
accomplishes this by updating the backward compatibility sections accomplishes this by updating the backward compatibility sections
found in TLS [TLS1.0][TLS1.1][TLS1.2]. found in TLS [TLS1.0][TLS1.1][TLS1.2].
1.1. Requirements Terminology 1.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. [RFC2119].
2. SSL 2.0 Deficiencies 2. SSL 2.0 Deficiencies
SSL version 2.0 [SSL2] deficiencies include: SSL version 2.0 [SSL2] deficiencies include the following:
o Message authentication uses MD5 [MD5]. Most security-aware users o Message authentication uses MD5 [MD5]. Most security-aware users
have already moved away from any use of MD5 have already moved away from any use of MD5 [RFC6151].
[I-D.turner-md5-seccon-update].
o Handshake messages are not protected. This permits a man-in-the- o Handshake messages are not protected. This permits a man-in-the-
middle to trick the client into picking a weaker cipher suite than middle to trick the client into picking a weaker cipher suite than
they would normally choose. it would normally choose.
o Message integrity and message encryption use the same key, which is o Message integrity and message encryption use the same key, which
a problem if the client and server negotiate a weak encryption is a problem if the client and server negotiate a weak encryption
algorithm. algorithm.
o Sessions can be easily terminated. A man-in-the-middle can easily 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 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. determine whether or not it was a legitimate end of the session.
3. Changes to TLS 3. Changes to TLS
Because of the deficiencies noted in the previous section: Because of the deficiencies noted in the previous section:
o TLS clients MUST NOT send the SSL version 2.0 compatible CLIENT- o TLS clients MUST NOT send the SSL version 2.0 compatible CLIENT-
HELLO message format. Clients MUST NOT send any client hello HELLO message format. Clients MUST NOT send any ClientHello
message which specifies a protocol version less than message that specifies a protocol version less than
{ 0x03, 0x00 }. As previously stated by the definitions of all { 0x03, 0x00 }. As previously stated by the definitions of all
previous versions of TLS, the client SHOULD specify the highest previous versions of TLS, the client SHOULD specify the highest
protocol version it supports. protocol version it supports.
o TLS servers MAY continue to accept CLIENT-HELLO messages in the o TLS servers MAY continue to accept ClientHello messages in the
version 2 CLIENT-HELLO format as specified in TLS 1.2 [RFC5246] version 2 CLIENT-HELLO format as specified in RFC 5246 [TLS1.2],
Appendix E.2. Note that this does not contradict the prohibition Appendix E.2. Note that this does not contradict the prohibition
against actually negotiating the use of SSL 2.0. against actually negotiating the use of SSL 2.0.
TLS Servers MUST NOT reply with a SSL 2.0 SERVER-HELLO with a o TLS servers MUST NOT reply with an SSL 2.0 SERVER-HELLO with a
protocol version which is less than { 0x03, 0x00 } and instead protocol version that is less than { 0x03, 0x00 } and instead MUST
MUST abort the connection, i.e., when the highest protocol version abort the connection, i.e., when the highest protocol version
offered by the client is { 0x02, 0x00 } the TLS connection will be offered by the client is { 0x02, 0x00 }, the TLS connection will
refused. be refused.
Note that the number of servers that support this above-mentioned Note that the number of servers that support this above-mentioned
"MAY accept" implementation option is declining, and the SSL 2.0 "MAY accept" implementation option is declining, and the SSL 2.0
CLIENT-HELLO precludes the use of TLS protocol enhancements that CLIENT-HELLO precludes the use of TLS protocol enhancements that
require TLS extensions. TLS extensions can only be sent as part of an require TLS extensions. TLS extensions can only be sent as part of
(Extended) ClientHello handshake message. an (Extended) ClientHello handshake message.
4. IANA Considerations
None.
5. Security Considerations 4. Security Considerations
This entire document is about security considerations. This entire document is about security considerations.
6. Acknowledgements 5. Acknowledgements
The idea for this document was inspired by discussions between Peter The idea for this document was inspired by discussions between Peter
Saint Andre, Simon Josefsson, and others on the XMPP mailing list. 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 We would also like to thank Michael D'Errico, Paul Hoffman, Nikos
Mavrogiannopoulos, Tom Petch, Yngve Pettersen, Marsh Ray, Martin Rex, Mavrogiannopoulos, Tom Petch, Yngve Pettersen, Marsh Ray, Martin Rex,
Yaron Sheffer, and Glen Zorn for their reviews and comments. Yaron Sheffer, and Glen Zorn for their reviews and comments.
7. References 6. References
7.1. Normative References 6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[TLS1.0] Dierks, T., and C. Allen, "The TLS Protocol Version [TLS1.0] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
1.0", RFC 2246, January 1999. RFC 2246, January 1999.
[TLS1.1] Dierks, T. and E. Rescorla, "The Transport Layer [TLS1.1] Dierks, T. and E. Rescorla, "The Transport Layer Security
Security (TLS) Protocol Version 1.1", RFC 4346, (TLS) Protocol Version 1.1", RFC 4346, April 2006.
April 2006.
[TLS1.2] Dierks, T. and E. Rescorla, "The Transport Layer [TLS1.2] Dierks, T. and E. Rescorla, "The Transport Layer Security
Security (TLS) Protocol Version 1.2", RFC 5246, (TLS) Protocol Version 1.2", RFC 5246, August 2008.
August 2008.
7.2. Informative References 6.2. Informative References
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
1321, April 1992. April 1992.
[SSL2] Hickman, Kipp, "The SSL Protocol", Netscape [SSL2] Hickman, Kipp, "The SSL Protocol", Netscape
Communications Corp., Feb 9, 1995. Communications Corp., Feb 9, 1995.
[I-D.turner-md5-seccon-update] Turner, S., and L. Chen, "Updated [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
Security Considerations for the MD5 Message-Digest for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
Algorithm", draft-turner-md5-seccon-update, work-in- RFC 6151, March 2011.
progress.
Authors' Addresses Authors' Addresses
Sean Turner Sean Turner
IECA, Inc. IECA, Inc.
3057 Nutley Street, Suite 106 3057 Nutley Street, Suite 106
Fairfax, VA 22031 Fairfax, VA 22031
USA USA
EMail: turners@ieca.com EMail: turners@ieca.com
Tim Polk Tim Polk
 End of changes. 42 change blocks. 
105 lines changed or deleted 88 lines changed or added

This html diff was produced by rfcdiff 1.41. The latest version is available from http://tools.ietf.org/tools/rfcdiff/