RFC7568
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/rfc7568.
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.
Contents
Introduction
Since it was released in 1996, the SSLv3 protocol RFC6101 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 RFC2246 in 1999, and subsequently TLS 1.1 in 2002 RFC4346 and 1.2 in 2006 RFC5246, 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 RFC6176. SSLv3 now follows.
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 RFC2119.
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 RFC5246 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.
SSLv3 Is Comprehensively Broken
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 RFC7366. 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 RFC7465.
This leaves SSLv3 with no suitable record protection mechanism.
Key Exchange
The SSLv3 key exchange is vulnerable to man-in-the-middle attacks when renegotiation RFC5746 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.
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 RFC3174 and MD5 RFC1321: these hash algorithms are considered weak and are being systematically replaced with stronger hash functions, such as SHA-256 [FIPS180-4].
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 RFC5246.
o Elliptic Curve Diffie-Hellman (ECDH) and Digital Signature
Algorithm (ECDSA) are added in RFC4492.
o Stateless session tickets RFC5077.
o A datagram mode of operation, DTLS RFC6347.
o Application-layer protocol negotiation RFC7301.
Security Considerations
This entire document aims to improve security by prohibiting the use of a protocol that is not secure.
References
Normative References
RFC2119 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>.
RFC2246 Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, DOI 10.17487/RFC2246, January 1999, <http://www.rfc-editor.org/info/rfc2246>.
RFC4346 Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, DOI 10.17487/RFC4346, April 2006, <http://www.rfc-editor.org/info/rfc4346>.
RFC5246 Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>.
RFC6101 Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101, DOI 10.17487/RFC6101, August 2011, <http://www.rfc-editor.org/info/rfc6101>.
RFC7366 Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
<http://www.rfc-editor.org/info/rfc7366>.
RFC7465 Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015,
<http://www.rfc-editor.org/info/rfc7465>.
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>.
RFC1321 Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<http://www.rfc-editor.org/info/rfc1321>.
RFC3174 Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm
1 (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001, <http://www.rfc-editor.org/info/rfc3174>.
RFC4492 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/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
RFC5077 Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption
without Server-Side State", RFC 5077,
DOI 10.17487/RFC5077, January 2008,
<http://www.rfc-editor.org/info/rfc5077>.
RFC5746 Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February
2010, <http://www.rfc-editor.org/info/rfc5746>.
RFC6176 Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March 2011, <http://www.rfc-editor.org/info/rfc6176>.
RFC6347 Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, <http://www.rfc-editor.org/info/rfc6347>.
RFC7301 Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer
Protocol Negotiation Extension", RFC 7301,
DOI 10.17487/RFC7301, July 2014,
<http://www.rfc-editor.org/info/rfc7301>.
[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.
Authors' Addresses
Richard Barnes Mozilla
EMail: [email protected]
Martin Thomson Mozilla
EMail: [email protected]
Alfredo Pironti INRIA
EMail: [email protected]
Adam Langley Google
EMail: [email protected]
