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Versions: (draft-rescorla-tls-renegotiation) 00 01 02 03 RFC 5746

Network Working Group                                        E. Rescorla
Internet-Draft                                                RTFM, Inc.
Intended status:  Standards Track                                 M. Ray
Expires:  May 30, 2010                                       S. Dispensa
                                                             PhoneFactor
                                                                N. Oskov
                                                               Microsoft
                                                       November 26, 2009


   Transport Layer Security (TLS) Renegotiation Indication Extension
                  draft-ietf-tls-renegotiation-01.txt

Abstract

   SSL and TLS renegotiation are vulnerable to an attack in which the
   attacker forms a TLS connection with the target server, injects
   content of his choice, and then splices in a new TLS connection from
   a client.  The server treats the client's initial TLS handshake as a
   renegotiation and thus believes that the initial data transmitted by
   the attacker is from the same entity as the subsequent client data.
   This draft defines a TLS extension to cryptographically tie
   renegotiations to the TLS connections they are being performed over,
   thus preventing this attack.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on May 30, 2010.




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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions Used In This Document . . . . . . . . . . . . . . . 4
   3.  Extension Definition  . . . . . . . . . . . . . . . . . . . . . 4
   4.  Renegotiation Protection Request Cipher Suite . . . . . . . . . 5
   5.  Requirements for Sending and Receiving  . . . . . . . . . . . . 5
   6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . . . 6
     6.1.  Client Considerations . . . . . . . . . . . . . . . . . . . 6
     6.2.  Server Considerations . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     10.1. Normative References  . . . . . . . . . . . . . . . . . . . 8
     10.2. Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8


































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1.  Introduction

   TLS [RFC5246] allows either the client or the server to initiate
   renegotiation--a new handshake which establishes new cryptographic
   parameters.  Unfortunately, although the new handshake is carried out
   over the protected channel established by the original handshake,
   there is no cryptographic connection between the two.  This creates
   the opportunity for an attack in which the attacker who can intercept
   a client's transport layer connection can inject traffic of his own
   as a prefix to the client's interaction with the server.  The attack
   proceeds as shown below:

   Client                        Attacker                        Server
   ------                        -------                         ------
                                     <----------- Handshake ---------->
                                     <======= Initial Traffic ========>
   <--------------------------  Handshake ============================>
   <======================== Client Traffic ==========================>

   To start the attack, the attacker forms a TLS connection to the
   server (perhaps in response to an initial intercepted connection from
   the client).  He then sends any traffic of his choice to the server.
   This may involve multiple requests and responses at the application
   layer, or may simply be a partial application layer request intended
   to prefix the client's data.  This traffic is shown with == to
   indicate it is encrypted.  He then allows the client's TLS handshake
   to proceed with the server.  The handshake is in the clear to the
   attacker but encrypted over the attacker's channel to the server.
   Once the handshake has completed, the client communicates with the
   server over the new channel.  The attacker cannot read this traffic,
   but the server believes that the initial traffic to and from the
   attacker is the same as that to and from the client.

   If certificate-based client authentication is used, the server will
   believe that the initial traffic corresponds to the authenticated
   client identity.  Even without certificate-based authentication, a
   variety of attacks may be possible in which the attacker convinces
   the server to accept data from it as data from the client.  For
   instance, if HTTPS [RFC2818] is in use with HTTP cookies [REF], the
   attacker may be able to generate a request of his choice validated by
   the client's cookie.

   This attack can be prevented by cryptographically binding
   renegotiation handshakes to the enclosing TLS channel, thus allowing
   the server to differentiate renegotiation from initial negotiation,
   as well as preventing renegotiations from being spliced in between
   connections.  An attempt by an attacker to inject himself as
   described above will result in a mismatch of the extension and can



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   thus be detected This document defines an extension that performs
   that cryptographic binding.  The extension described here is similar
   to that used for TLS Channel Bindings
   [I-D.altman-tls-channel-bindings].


2.  Conventions Used In This Document

   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].


3.  Extension Definition

   This document defines a new TLS extension:  "renegotiation_info",
   which contains a cryptographic binding to the enclosing TLS
   connection (if any) for which the renegotiation is being performed.
   The "extension data" field of this extension contains a
   "Renegotiation_Info" structure:

             struct {
               opaque renegotiated_connection<0..255>;
             } Renegotiation_Info;

   The contents of this extension are specified as follows.

   o  If this is the initial handshake for a connection, then the
      "renegotiated_connection" field is of zero length in both the
      ClientHello and the ServerHello.  Thus, the entire encoding of the
      extension is:  ff 01 00 01 00.  The first two octets represent the
      extension type, the third and fourth octet the length of the
      extension itself, and the final octet the zero length byte for the
      "renegotiated_connection" field.
   o  For ClientHellos which are renegotiating, this field contains the
      verify_data from the Finished message sent by the client on the
      immediately previous handshake.  For current versions of TLS, this
      will be a 12-byte value.  Note that this value is the "tls-unique"
      channel binding from [I-D.altman-tls-channel-bindings]
   o  For ServerHellos which are renegotiating, this field contains the
      concatenation of the verify_data values sent by the client and the
      server (in that order) on the immediately previous handshake.  For
      current versions of TLS, this will be a 24-byte value.

   The above rules apply even when TLS resumption is used.

   Upon receipt of the "renegotiation_info" extension, both client and
   server implementations which support the extension MUST verify that



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   it contains the correct contents as specified above.  If the contents
   are incorrect, then it MUST generate a fatal "handshake_failure"
   alert and terminate the connection.  This allows two implementations
   both of which support the extension to safely renegotiate without
   fear of the above attack.


4.  Renegotiation Protection Request Cipher Suite

   Both the SSLv3 and TLS 1.0/TLS1.1 specifications require
   implementations to ignore data following the ClientHello (i.e.,
   extensions) if they do not understand it.  However, some SSLv3 and
   TLS 1.0 implementations incorrectly fail the handshake in such case.
   This means that clients which offer "renegotiation_info" may find
   handshake failures.  In order to enhance compatibility with such
   servers, this document defines a second signalling mechanism via a
   special TLS cipher suite "TLS_RENEGO_PROTECTION_REQUEST", with code
   point 0xNN, 0xMM.  This cipher suite has exactly the same semantics
   as an empty "renegotiation_info" extension.  Because servers
   ordinarily ignore unknown cipher suites, this cipher suite can be
   added safely on any initial handshake, including SSLv2 backward
   compatibility handshakes.

   Servers MUST treat receipt of TLS_RENEGO_PROTECTION_REQUEST exactly
   as if the client had sent an empty "renegotiation_info" extension and
   respond with their own "renegotiation_info" extension.  This is an
   explicit exception to the RFC 5246 Section 7.4.1.4 prohibition on the
   server sending unsolicited extensions and is only allowed because the
   client is signaling its willingness to receive the extension via the
   the TLS_RENEGO_PROTECTION_REQUEST cipher suite.  TLS implementations
   MUST continue to comply with 7.4.1.4 for all other extensions.
   Servers MUST NOT select this cipher suite in any handshake, as it
   does not correspond to any valid set of algorithms.

   Because this cipher suite is equivalent to an empty
   "renegotiation_info" extension, only renegotiation_info" may be used
   rehandshakes.

   Note that a minimal client which does not support renegotiation at
   all can simply use this cipher suite in all initial handshakes.  Any
   compliant server will reject any (apparent) attempt at renegotiation
   by such a client.  Clients which do support renegotiation MUST
   implement Section 3 as well.


5.  Requirements for Sending and Receiving

   TLS clients which support this draft MUST generate either the



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   "renegotiation_info" extension or the TLS_RENEGO_PROTECTION_REQUEST
   cipher suite with every ClientHello.

   TLS servers which support this draft MUST generate the
   "renegotiation_info" extension in the ServerHello in response to any
   client which offers either "renegotiation_info" or
   TLS_RENEGO_PROTECTION_REQUEST in the ClientHello.


6.  Backward Compatibility

   Existing implementations which do not support this extension are
   widely deployed and in general must interoperate with newer
   implementations which do support it.  This section describes
   considerations for backward compatible interoperation.

6.1.  Client Considerations

   If a client offers the "renegotiation_info" extension or the
   TLS_RENEGO_PROTECTION_REQUEST cipher suite and the server does not
   reply with "renegotiation_info" in the ServerHello, then this
   indicates that the server either does not support secure
   renegotiation or is unwilling to use it.  Because the above attack
   looks like a single handshake to the client, the client cannot
   determine whether the connection is under attack or not.  Note,
   however, that merely because the server does not acknowledge the
   extension does not mean that it is vulnerable; it might choose to
   reject all rehandshakes and simply not signal it.  However, it is not
   possible for the client to determine purely via TLS mechanisms
   whether this is the case or not.

   If clients wish to ensure that such attacks are impossible, they MUST
   terminate the connection immediately upon failure to receive the
   extension without completing the handshake.  However, it is expected
   that many TLS servers that do not support renegotiation (and thus are
   not vulnerable) will not support this extension either, so in
   general, such behavior would not work well.

6.2.  Server Considerations

   If the client does not offer the "renegotiation_info" extension or
   the TLS_RENEGO_PROTECTION_REQUEST cipher suite then this indicates
   that the client does not support secure renegotiation or is unwilling
   to use it.  However, because the above attack looks like two
   handshakes to the server, the server can safely continue the
   connection as long as it does not allow the client to rehandshake.
   If servers wish to ensure that such attacks are impossible they MUST
   NOT allow clients who do not offer the "renegotiation_info" extension



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   to renegotiate with them and SHOULD respond to such requests with a
   "no_renegotiation" alert [RFC 5246 requires this alert to be at the
   "warning" level.]  Servers SHOULD follow this behavior.

   In order to enable clients to probe, even servers which do not
   support renegotiation SHOULD implement the minimal version of the
   extension described in this document for initial handshakes, thus
   signalling that they have been upgraded.


7.  Security Considerations

   The extension described in this document prevents an attack on TLS.
   If this extension is not used, TLS renegotiation is subject to an
   attack in which the attacker can inject their own conversation with
   the TLS server as a prefix of the client's conversation.  This attack
   is invisible to the client and looks like an ordinary renegotiation
   to the server.  The extension defined in this document allows
   renegotiation to be performed safely.  Servers SHOULD NOT allow
   clients to renegotiate without using this extension.

   While this extension mitigates the man-in-the-middle attack described
   in the overview, it does not resolve all possible problems an
   application may face if it is unaware of renegotiation.  It is
   possible that the authenticated identity of the server or client may
   change as a result of renegotiation.

   By default, TLS implementations conforming to this document MUST
   verify that once the peer has been identified and authenticated
   within the TLS handshake, the identity does not change on subsequent
   renegotiations.  For certificate based cipher suites, this means
   bitwise equality of the end-entity certificate.  If the other end
   attempts to authenticate with a different identity, the renegotiation
   MUST fail.  If the server_name extension is used, it MUST NOT change
   when doing renegotiation.

   A TLS library MAY provide a means for the application to allow
   identity and/or server_name changes across renegotiations, in which
   case the application is responsible for tracking the identity
   associated with data it is processing.  This may require additional
   API facilities in the TLS library.


8.  IANA Considerations

   IANA [shall add/has added] the extension code point XXX [We request
   0xff01, which has been used for prototype implementations] for the
   "renegotiation_info" extension to the TLS ExtensionType values



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   registry.

   IANA [shall add/has added] TLS cipher suite number 0xNN,0xMM with
   name TLS_RENEGO_PROTECTION_REQUEST to the TLS Cipher Suite registry.


9.  Acknowledgements

   This vulnerability was originally discovered by Marsh Ray. The
   general concept behind the extension described here was independently
   invented by Steve Dispensa, Nasko Oskov, and Eric Rescorla with
   refinements from Nelson Bolyard, Pasi Eronen, Mike D'Errico, Bodo
   Moeller, Martin Rex (who defined TLS_RENEGO_PROTECTION_REQUEST),
   Jesse Walker, Nico Williams and other members of the the Project
   Mogul team and the TLS WG.  [Note:  if you think your name should be
   here, please speak up.]


10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

10.2.  Informative References

   [I-D.altman-tls-channel-bindings]
              Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", draft-altman-tls-channel-bindings-07 (work in
              progress), October 2009.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.


Authors' Addresses

   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303
   USA

   Email:  ekr@rtfm.com




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   Marsh Ray
   PhoneFactor
   7301 W 129th Street
   Overland Park, KS  66213
   USA

   Email:  marsh@extendedsubset.com


   Steve Dispensa
   PhoneFactor
   7301 W 129th Street
   Overland Park, KS  66213
   USA

   Email:  dispensa@phonefactor.com


   Nasko Oskov
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   USA

   Email:  nasko.oskov@microsoft.com


























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