TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks
RFC 7507

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/rfc7507.

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.

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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.)

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   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 [RFC2246],
   TLS 1.1 [RFC4346], and TLS 1.2 [RFC5246], and to implementations of
   DTLS 1.0 [RFC4347] and DTLS 1.2 [RFC6347].  (It is particularly
   relevant if the TLS implementations also include support for
   predecessor protocol SSL 3.0 [RFC6101].)  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 [RFC2119].

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 [RFC5246]:

        enum {
          /* ... */
          inappropriate_fallback(86),
          /* ... */
          (255)
        } AlertDescription;

   This alert is only generated by servers, as described in Section 3.
   It is always fatal.

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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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   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 [RFC5246].)

   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.

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   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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         +-------+------------------------+---------+-----------+
         | Value |      Description       | DTLS-OK | Reference |
         +-------+------------------------+---------+-----------+
         |   86  | inappropriate_fallback |    Y    |  RFC 7507 |
         +-------+------------------------+---------+-----------+

                    Addition to the TLS Alert Registry

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, 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, April 2006,
              <http://www.rfc-editor.org/info/rfc4346>.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006,
              <http://www.rfc-editor.org/info/rfc4347>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012,
              <http://www.rfc-editor.org/info/rfc6347>.

8.2.  Informative References

   [RFC6101]  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/rfc6101>.

   [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.

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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

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