Internet Engineering Task Force (IETF) R. Barnes
Request for Comments: 7568 M. Thomson
Updates: 5246 Mozilla
Category: Standards Track A. Pironti
ISSN: 2070-1721 INRIA
A. Langley
Google
June 2015
Deprecating Secure Sockets Layer Version 3.0
Abstract
The Secure Sockets Layer version 3.0 (SSLv3), as specified in RFC
6101, is not sufficiently secure. This document requires that SSLv3
not be used. The replacement versions, in particular, Transport
Layer Security (TLS) 1.2 (RFC 5246), are considerably more secure and
capable protocols.
This document updates the backward compatibility section of RFC 5246
and its predecessors to prohibit fallback to SSLv3.
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
http://www.rfc-editor.org/info/RFC 7568.
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Copyright Notice
Copyright (c) 2015 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
(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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Do Not Use SSL Version 3.0 . . . . . . . . . . . . . . . . . 3
4. SSLv3 Is Comprehensively Broken . . . . . . . . . . . . . . . 3
4.1. Record Layer . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Key Exchange . . . . . . . . . . . . . . . . . . . . . . 4
4.3. Custom Cryptographic Primitives . . . . . . . . . . . . . 4
5. Limited Capabilities . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Since it was released in 1996, the SSLv3 protocol [RFC 6101] has been
subject to a long series of attacks, both on its key exchange
mechanism and on the encryption schemes it supports. Despite being
replaced by TLS 1.0 [RFC 2246] in 1999, and subsequently TLS 1.1 in
2002 [RFC 4346] and 1.2 in 2006 [RFC 5246], availability of these
replacement versions has not been universal. As a result, many
implementations of TLS have permitted the negotiation of SSLv3.
The predecessor of SSLv3, SSL version 2, is no longer considered
sufficiently secure [RFC 6176]. SSLv3 now follows.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
3. Do Not Use SSL Version 3.0
SSLv3 MUST NOT be used. Negotiation of SSLv3 from any version of TLS
MUST NOT be permitted.
Any version of TLS is more secure than SSLv3, though the highest
version available is preferable.
Pragmatically, clients MUST NOT send a ClientHello with
ClientHello.client_version set to {03,00}. Similarly, servers MUST
NOT send a ServerHello with ServerHello.server_version set to
{03,00}. Any party receiving a Hello message with the protocol
version set to {03,00} MUST respond with a "protocol_version" alert
message and close the connection.
Historically, TLS specifications were not clear on what the record
layer version number (TLSPlaintext.version) could contain when
sending ClientHello. Appendix E of [RFC 5246] notes that
TLSPlaintext.version could be selected to maximize interoperability,
though no definitive value is identified as ideal. That guidance is
still applicable; therefore, TLS servers MUST accept any value
{03,XX} (including {03,00}) as the record layer version number for
ClientHello, but they MUST NOT negotiate SSLv3.
4. SSLv3 Is Comprehensively Broken
4.1. Record Layer
The non-deterministic padding used in the Cipher Block Chaining (CBC)
construction of SSLv3 trivially permits the recovery of plaintext
[POODLE]. More generally, the CBC modes of SSLv3 use a flawed MAC-
then-encrypt construction that has subsequently been replaced in TLS
versions [RFC 7366]. Unfortunately, the mechanism to correct this
flaw relies on extensions: a feature added in TLS 1.0. SSLv3 cannot
be updated to correct this flaw in the same way.
The flaws in the CBC modes in SSLv3 are mirrored by the weakness of
the stream ciphers it defines. Of those defined, only RC4 is
currently in widespread use. RC4, however, exhibits serious biases
and is also no longer fit for use [RFC 7465].
This leaves SSLv3 with no suitable record protection mechanism.
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4.2. Key Exchange
The SSLv3 key exchange is vulnerable to man-in-the-middle attacks
when renegotiation [RFC 5746] or session resumption [TRIPLE-HS] are
used. Each flaw has been fixed in TLS by means of extensions.
Again, SSLv3 cannot be updated to correct these flaws.
4.3. Custom Cryptographic Primitives
SSLv3 defines custom constructions for Pseudorandom Function (PRF),
Hashed Message Authentication Code (HMAC), and digital signature
primitives. Such constructions lack the deep cryptographic scrutiny
that standard constructions used by TLS have received. Furthermore,
all SSLv3 primitives rely on SHA-1 [RFC 3174] and MD5 [RFC 1321]: these
hash algorithms are considered weak and are being systematically
replaced with stronger hash functions, such as SHA-256 [FIPS180-4].
5. Limited Capabilities
SSLv3 is unable to take advantage of the many features that have been
added to recent TLS versions. This includes the features that are
enabled by ClientHello extensions, which SSLv3 does not support.
Though SSLv3 can benefit from new cipher suites, it cannot benefit
from new cryptographic modes and features. Of these, the following
are particularly prominent:
o Authenticated Encryption with Additional Data (AEAD) modes are
added in [RFC 5246].
o Elliptic Curve Diffie-Hellman (ECDH) and Digital Signature
Algorithm (ECDSA) are added in [RFC 4492].
o Stateless session tickets [RFC 5077].
o A datagram mode of operation, DTLS [RFC 6347].
o Application-layer protocol negotiation [RFC 7301].
6. Security Considerations
This entire document aims to improve security by prohibiting the use
of a protocol that is not secure.
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7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, DOI 10.17487/RFC 2246, January 1999,
<http://www.rfc-editor.org/info/RFC 2246>.
[RFC 4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC 4346, April 2006,
<http://www.rfc-editor.org/info/RFC 4346>.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC 5246, August 2008,
<http://www.rfc-editor.org/info/RFC 5246>.
[RFC 6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
DOI 10.17487/RFC 6101, August 2011,
<http://www.rfc-editor.org/info/RFC 6101>.
[RFC 7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, DOI 10.17487/RFC 7366, September 2014,
<http://www.rfc-editor.org/info/RFC 7366>.
[RFC 7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC 7465, February 2015,
<http://www.rfc-editor.org/info/RFC 7465>.
7.2. Informative References
[FIPS180-4] U.S. National Institute of Standards and Technology,
"Secure Hash Standard", FIPS 180-4, March 2012.
[POODLE] Moeller, B., "This POODLE bites: exploiting the SSL 3.0
fallback", October 2014,
<http://googleonlinesecurity.blogspot.com/2014/10/
this-poodle-bites-exploiting-ssl-30.html>.
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[RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC 1321, April 1992,
<http://www.rfc-editor.org/info/RFC 1321>.
[RFC 3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm
1 (SHA1)", RFC 3174, DOI 10.17487/RFC 3174, September
2001, <http://www.rfc-editor.org/info/RFC 3174>.
[RFC 4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and
B. Moeller, "Elliptic Curve Cryptography (ECC) Cipher
Suites for Transport Layer Security (TLS)", RFC 4492, DOI
10.17487/RFC 4492, May 2006,
<http://www.rfc-editor.org/info/RFC 4492>.
[RFC 5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption
without Server-Side State", RFC 5077,
DOI 10.17487/RFC 5077, January 2008,
<http://www.rfc-editor.org/info/RFC 5077>.
[RFC 5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC 5746, February
2010, <http://www.rfc-editor.org/info/RFC 5746>.
[RFC 6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC 6176, March
2011, <http://www.rfc-editor.org/info/RFC 6176>.
[RFC 6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC 6347,
January 2012, <http://www.rfc-editor.org/info/RFC 6347>.
[RFC 7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer
Protocol Negotiation Extension", RFC 7301,
DOI 10.17487/RFC 7301, July 2014,
<http://www.rfc-editor.org/info/RFC 7301>.
[TRIPLE-HS] Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
A., and P-Y. Strub, "Triple Handshakes and Cookie
Cutters: Breaking and Fixing Authentication over TLS",
IEEE Symposium on Security and Privacy, 2014.
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Authors' Addresses
Richard Barnes
Mozilla
EMail: rlb@ipv.sx
Martin Thomson
Mozilla
EMail: martin.thomson@gmail.com
Alfredo Pironti
INRIA
EMail: alfredo@pironti.eu
Adam Langley
Google
EMail: agl@google.com
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