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IETF RFC 7568



Last modified on Thursday, June 25th, 2015

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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.















Barnes, et al.               Standards Track                 PAGE 1 top


RFC 7568 SSLv3 Is Not Secure June 2015 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. Barnes, et al. Standards Track PAGE 2 top

RFC 7568 SSLv3 Is Not Secure June 2015 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. Barnes, et al. Standards Track PAGE 3 top

RFC 7568 SSLv3 Is Not Secure June 2015 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. Barnes, et al. Standards Track PAGE 4 top

RFC 7568 SSLv3 Is Not Secure June 2015 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>. Barnes, et al. Standards Track PAGE 5 top

RFC 7568 SSLv3 Is Not Secure June 2015 [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. Barnes, et al. Standards Track PAGE 6 top

RFC 7568 SSLv3 Is Not Secure June 2015 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 Barnes, et al. Standards Track PAGE 7 top

RFC TOTAL SIZE: 13489 bytes PUBLICATION DATE: Thursday, June 25th, 2015 LEGAL RIGHTS: The IETF Trust (see BCP 78)


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