Deprecating SSLv3: Deprecating Secure Sockets Layer Version 3.0
Why SSL version 3.0 is broken and must not be used, with replacements.
Barnes, R., Thomson, M., Pironti, A., and A. Langley, "Deprecating Secure Sockets Layer Version 3.0", RFC 7568, DOI 10.17487/RFC7568, June 2015, <https://www.rfc-editor.org/info/rfc7568>.
@misc{rfc7568,
series = {Request for Comments},
number = 7568,
howpublished = {RFC 7568},
publisher = {RFC Editor},
doi = {10.17487/RFC7568},
url = {https://www.rfc-editor.org/info/rfc7568},
author = {R. Barnes and M. Thomson and A. Pironti and A. Langley},
title = {{Deprecating Secure Sockets Layer Version 3.0}},
pagetotal = 7,
year = 2015,
month = jun,
} RFC text
Contents
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.
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 [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.
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 [RFC2119].
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 [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.
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 [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.
4.2. 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.
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 [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].
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 [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].
6. Security Considerations
This entire document aims to improve security by prohibiting the use
of a protocol that is not secure.
7. References
7.1. 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>.
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>.
[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: 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
AI ELI5
On this page
§1Why SSLv3 Must Be Retired
SSL version 3.0 (SSLv3), first released in 1996, has been subject to a long series of attacks on both its key exchange mechanism and its encryption schemes. Despite being replaced by TLS 1.0 in 1999, and then TLS 1.1 (2002) and TLS 1.2 (2006), availability of these newer versions was not universal. Many TLS implementations permitted negotiation back to SSLv3 for backward compatibility with older systems.
SSLv3's predecessor, SSL version 2, was already prohibited. Now SSLv3 follows. Any version of TLS is more secure than SSLv3; the highest version available should be preferred.
SSLv3 is an old security lock from 1996. It has been broken by attackers many times. Even though better locks were invented in 1999, some systems kept accepting the old lock. Now everyone must stop using it.
§2Record Layer Vulnerabilities
SSLv3's record layer uses Cipher Block Chaining (CBC) mode with non-deterministic padding, which trivially permits recovery of plaintext through the POODLE attack. The flaw is fundamental: SSLv3 uses a flawed MAC-then-encrypt construction that was later replaced in TLS versions using extensions (a feature added in TLS 1.0).
SSLv3 cannot be updated to fix this flaw in the same way because it does not support extensions. The stream ciphers defined in SSLv3 are also weak: of those defined, only RC4 is in widespread use, but RC4 exhibits serious biases and is no longer fit for use. This leaves SSLv3 with no suitable record protection mechanism.
SSLv3's way of scrambling data (CBC mode) has a hole that lets attackers peek at the messages. The newer TLS versions fixed this, but SSLv3 cannot be fixed the same way.
§3Key Exchange and Cryptographic Flaws
SSLv3's key exchange is vulnerable to man-in-the-middle attacks when renegotiation or session resumption are used. Each flaw has been fixed in TLS by means of extensions, but SSLv3 cannot be updated to correct these flaws.
SSLv3 also defines custom cryptographic constructions for the Pseudorandom Function (PRF), Hashed Message Authentication Code (HMAC), and digital signature primitives. These custom constructions lack the deep cryptographic scrutiny that standard constructions used by TLS have received. Critically, all SSLv3 primitives rely on SHA-1 and MD5, both considered weak and being replaced with stronger algorithms like SHA-256.
SSLv3 uses old, custom ways to mix up keys and verify messages that experts have found weak. Newer TLS uses standard methods that have been studied much more carefully.
§4What Clients and Servers Must Do
Clients MUST NOT send a ClientHello with client_version set to {03,00}. Servers MUST NOT send a ServerHello with server_version set to {03,00}. Any party receiving a Hello message with protocol version {03,00} MUST respond with a protocol_version alert and close the connection.
Historically, TLS specifications did not specify what the record layer version number could contain when sending ClientHello. Servers MUST accept any value {03,XX} (including {03,00}) as the record layer version for ClientHello to maintain interoperability with clients, but they MUST NOT negotiate SSLv3 itself. This allows clients with legacy record layer version numbers to connect, but guarantees they will use a secure TLS version.
If a client or server tries to use SSLv3, the other side must say no and hang up. But servers still accept old-style messages that might claim to be SSLv3, as long as the actual conversation uses a newer, safer version.
§5Why TLS Versions Are Better
SSLv3 is unable to take advantage of the many features added to recent TLS versions. TLS 1.0 introduced extensions, which allow the protocol to be extended without breaking compatibility. TLS 1.2 added Authenticated Encryption with Additional Data (AEAD) modes, Elliptic Curve Diffie-Hellman (ECDH) and Elliptic Curve Digital Signature Algorithm (ECDSA) for stronger key exchange and signatures.
Later versions added stateless session tickets for session resumption without server-side state, DTLS as a datagram mode for UDP-based protocols, and Application-Layer Protocol Negotiation to let clients and servers agree on which application protocol to use (such as HTTP/2). SSLv3 can benefit from new cipher suites but cannot benefit from these modern cryptographic modes and features.
Newer TLS versions have many new features like AEAD encryption and elliptic curves. SSLv3 is so old it cannot even use these, so it is stuck with weak tools.
§6Status and Deployment
RFC 7568 was published in June 2015 as a Standards Track RFC. It updates the backward-compatibility section of RFC 5246 and predecessors to prohibit SSLv3. The RFC was itself updated by RFC 8996, which deprecated TLS 1.0 and 1.1 as well.
By the time this RFC was published, major browsers and servers had already begun rejecting SSLv3. The POODLE attack, publicized in October 2014, accelerated deprecation. Today, SSLv3 support is rare in modern implementations and is blocked by major platforms and browsers.
This rule came out in 2015, right after attackers found a big hole in SSLv3. Since then, almost all software has stopped supporting it.