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

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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/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.

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