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IETF RFC 7507
Last modified on Saturday, April 25th, 2015
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Internet Engineering Task Force (IETF) B. Moeller
Request for Comments: 7507 A. Langley
Updates: 2246, 4346, 4347, 5246, 6347 Google
Category: Standards Track April 2015
ISSN: 2070-1721
TLS Fallback Signaling Cipher Suite Value (SCSV)
for Preventing Protocol Downgrade Attacks
 Abstract
This document defines a Signaling Cipher Suite Value (SCSV) that
prevents protocol downgrade attacks on the Transport Layer Security
(TLS) and Datagram Transport Layer Security (DTLS) protocols. It
updates RFCs 2246, 4346, 4347, 5246, and 6347. Server update
considerations are included.
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 7507.
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.
Moeller & Langley Standards Track PAGE 1 
RFC 7507 TLS Fallback SCSV April 2015
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Protocol Values . . . . . . . . . . . . . . . . . . . . . . . 3
3. Server Behavior . . . . . . . . . . . . . . . . . . . . . . . 4
4. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 4
5. Operational Considerations . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
To work around interoperability problems with legacy servers, many
TLS client implementations do not rely on the TLS protocol version
negotiation mechanism alone but will intentionally reconnect using a
downgraded protocol if initial handshake attempts fail. Such clients
may fall back to connections in which they announce a version as low
as TLS 1.0 (or even its predecessor, Secure Socket Layer (SSL) 3.0)
as the highest supported version.
While such fallback retries can be a useful last resort for
connections to actual legacy servers, there's a risk that active
attackers could exploit the downgrade strategy to weaken the
cryptographic security of connections. Also, handshake errors due to
network glitches could similarly be misinterpreted as interaction
with a legacy server and result in a protocol downgrade.
All unnecessary protocol downgrades are undesirable (e.g., from TLS
1.2 to TLS 1.1, if both the client and the server actually do support
TLS 1.2); they can be particularly harmful when the result is loss of
the TLS extension feature by downgrading to SSL 3.0. This document
defines an SCSV that can be employed to prevent unintended protocol
downgrades between clients and servers that comply with this document
by having the client indicate that the current connection attempt is
merely a fallback and by having the server return a fatal alert if it
detects an inappropriate fallback. (The alert does not necessarily
indicate an intentional downgrade attack, since network glitches too
could result in inappropriate fallback retries.)
Moeller & Langley Standards Track PAGE 2
RFC 7507 TLS Fallback SCSV April 2015
The fallback SCSV defined in this document is not a suitable
substitute for proper TLS version negotiation. TLS implementations
need to properly handle TLS version negotiation and extensibility
mechanisms to avoid the security issues and connection delays
associated with fallback retries.
This specification applies to implementations of TLS 1.0 [RFC 2246],
TLS 1.1 [RFC 4346], and TLS 1.2 [RFC 5246], and to implementations of
DTLS 1.0 [RFC 4347] and DTLS 1.2 [RFC 6347]. (It is particularly
relevant if the TLS implementations also include support for
predecessor protocol SSL 3.0 [RFC 6101].) It can be applied similarly
to later protocol versions.
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].
2. Protocol Values
This document defines a new TLS cipher suite value:
TLS_FALLBACK_SCSV {0x56, 0x00}
This is an SCSV, i.e., it does not actually correspond to a suite of
cryptosystems, and it can never be selected by the server in the
handshake; rather, its presence in the Client Hello message serves as
a backwards-compatible signal from the client to the server.
This document also allocates a new alert value in the TLS Alert
Registry [RFC 5246]:
enum {
/* ... */
inappropriate_fallback(86),
/* ... */
(255)
} AlertDescription;
This alert is only generated by servers, as described in Section 3.
It is always fatal.
Moeller & Langley Standards Track PAGE 3
RFC 7507 TLS Fallback SCSV April 2015
3. Server Behavior
This section specifies server behavior when receiving the
TLS_FALLBACK_SCSV cipher suite from a client in
ClientHello.cipher_suites.
o If TLS_FALLBACK_SCSV appears in ClientHello.cipher_suites and the
highest protocol version supported by the server is higher than
the version indicated in ClientHello.client_version, the server
MUST respond with a fatal inappropriate_fallback alert (unless it
responds with a fatal protocol_version alert because the version
indicated in ClientHello.client_version is unsupported). The
record layer version number for this alert MUST be set to either
ClientHello.client_version (as it would for the Server Hello
message if the server was continuing the handshake) or to the
record layer version number used by the client.
o Otherwise (either TLS_FALLBACK_SCSV does not appear or it appears
and the client's protocol version is at least the highest protocol
version supported by the server), the server proceeds with the
handshake as usual.
(A protocol version is supported by the server if, in response to
appropriate Client Hello messages, the server would use it for
ServerHello.server_version. If a particular protocol version is
implemented but completely disabled by server settings, it is not
considered supported. For example, if the implementation's highest
protocol version is TLS 1.2 but the server operator has disabled this
version, a TLS 1.1 Client Hello with TLS_FALLBACK_SCSV does not
warrant responding with an inappropriate_fallback alert.)
4. Client Behavior
The TLS_FALLBACK_SCSV cipher suite value is meant for use by clients
that repeat a connection attempt with a downgraded protocol (perform
a "fallback retry") in order to work around interoperability problems
with legacy servers.
o If a client sends a ClientHello.client_version containing a lower
value than the latest (highest-valued) version supported by the
client, it SHOULD include the TLS_FALLBACK_SCSV cipher suite value
in ClientHello.cipher_suites; see Section 6 for security
considerations for this recommendation. (The client SHOULD put
TLS_FALLBACK_SCSV after all cipher suites that it actually intends
to negotiate.)
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RFC 7507 TLS Fallback SCSV April 2015
o As an exception to the above, when a client intends to resume a
session and sets ClientHello.client_version to the protocol
version negotiated for that session, it MUST NOT include
TLS_FALLBACK_SCSV in ClientHello.cipher_suites. (In this case, it
is assumed that the client already knows the highest protocol
version supported by the server: see Appendix E.1 of [RFC 5246].)
o If a client sets ClientHello.client_version to its highest
supported protocol version, it MUST NOT include TLS_FALLBACK_SCSV
in ClientHello.cipher_suites.
(A protocol version is supported by the client if the client normally
attempts to use it in handshakes. If a particular protocol version
is implemented but completely disabled by client settings, it is not
considered supported. For example, if the implementation's highest
protocol version is TLS 1.2 but the user has disabled this version, a
TLS 1.1 handshake is expected and does not warrant sending
TLS_FALLBACK_SCSV.)
Fallback retries could be caused by events such as network glitches,
and a client including TLS_FALLBACK_SCSV in ClientHello.cipher_suites
may receive an inappropriate_fallback alert in response, indicating
that the server supports a higher protocol version. Thus, if a
client intends to use retries to work around network glitches, it
should then retry with the highest version it supports.
If a client keeps track of the highest protocol version apparently
supported by a particular server for use in
ClientHello.client_version later, then if the client receives an
inappropriate_fallback alert from that server, it MUST clear the
memorized highest supported protocol version. (Without the alert, it
is a good idea -- but outside of the scope of this document -- for
clients to clear that state after a timeout since the server's
highest protocol version could change over time.)
For clients that use client-side TLS False Start [false-start], it is
important to note that the TLS_FALLBACK_SCSV mechanism cannot protect
the first round of application data sent by the client: refer to the
Security Considerations (Section 6) of [false-start].
5. Operational Considerations
Updating legacy server clusters to simultaneously add support for
newer protocol versions and support for TLS_FALLBACK_SCSV can have
complications if the legacy server implementation is not "version-
tolerant" (cannot properly handle Client Hello messages for newer
protocol versions): fallback retries required for interoperability
with old server nodes might be rejected by updated server nodes.
Moeller & Langley Standards Track PAGE 5
RFC 7507 TLS Fallback SCSV April 2015
Updating the server cluster in two consecutive steps makes this safe:
first, update the server software but leave the highest supported
version unchanged (by disabling newer versions in server settings);
then, after all legacy (version-intolerant) implementations have been
removed, change server settings to allow new protocol versions.
6. Security Considerations
Section 4 does not require client implementations to send
TLS_FALLBACK_SCSV in any particular case, it merely recommends it;
behavior can be adapted according to the client's security needs. It
is important to remember that omitting TLS_FALLBACK_SCSV enables
downgrade attacks, so implementors must take into account whether the
protocol version given by ClientHello.client_version still provides
an acceptable level of protection. For example, during the initial
deployment of a new protocol version (when some interoperability
problems may have to be expected), smoothly falling back to the
previous protocol version in case of problems may be preferable to
potentially not being able to connect at all: so TLS_FALLBACK_SCSV
could be omitted for this particular protocol downgrade step.
However, it is strongly recommended to send TLS_FALLBACK_SCSV when
downgrading to SSL 3.0 as the Cipher Block Chaining (CBC) cipher
suites in SSL 3.0 have weaknesses that cannot be addressed by
implementation workarounds like the remaining weaknesses in later
(TLS) protocol versions.
7. IANA Considerations
IANA has added TLS cipher suite number 0x56,0x00 with the name
TLS_FALLBACK_SCSV to the TLS Cipher Suite Registry and alert number
86 with the name inappropriate_fallback to the TLS Alert Registry, as
shown below. The registries are available from
<http://www.iana.org/assignments/tls-parameters>.
+-----------+-------------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-----------+-------------------+---------+-----------+
| 0x56,0x00 | TLS_FALLBACK_SCSV | Y | RFC 7507 |
+-----------+-------------------+---------+-----------+
Addition to the TLS Cipher Suite Registry
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RFC 7507 TLS Fallback SCSV April 2015
+-------+------------------------+---------+-----------+
| Value | Description | DTLS-OK | Reference |
+-------+------------------------+---------+-----------+
| 86 | inappropriate_fallback | Y | RFC 7507 |
+-------+------------------------+---------+-----------+
Addition to the TLS Alert Registry
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, 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, 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, April 2006,
<http://www.rfc-editor.org/info/RFC 4346>.
[RFC 4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006,
<http://www.rfc-editor.org/info/RFC 4347>.
[RFC 5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/RFC 5246>.
[RFC 6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012,
<http://www.rfc-editor.org/info/RFC 6347>.
8.2. Informative References
[RFC 6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
August 2011, <http://www.rfc-editor.org/info/RFC 6101>.
[false-start]
Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", Work in Progress,
draft-bmoeller-tls-falsestart-01, November 2014.
Moeller & Langley Standards Track PAGE 7
RFC 7507 TLS Fallback SCSV April 2015
Acknowledgements
This specification was inspired by an earlier proposal by Eric
Rescorla. We also thank Daniel Kahn Gillmor, Joe Saloway, Brian
Smith, Martin Thomson, and others in the TLS Working Group for their
feedback and suggestions.
Authors' Addresses
Bodo Moeller
Google Switzerland GmbH
Brandschenkestrasse 110
Zurich 8002
Switzerland
EMail: bmoeller@acm.org
Adam Langley
Google Inc.
345 Spear St
San Francisco, CA 94105
United States
EMail: agl@google.com
Moeller & Langley Standards Track PAGE 8
RFC TOTAL SIZE: 17165 bytes
PUBLICATION DATE: Saturday, April 25th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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