Opportunistic Security for HTTP/2
draft-ietf-httpbis-http2-encryption-11

HTTP Working Group                                         M. Nottingham
Internet-Draft
Intended status: Experimental                                 M. Thomson
Expires: September 18, 2017                                      Mozilla
                                                          March 17, 2017

                   Opportunistic Security for HTTP/2
                 draft-ietf-httpbis-http2-encryption-11

Abstract

   This document describes how "http" URIs can be accessed using
   Transport Layer Security (TLS) and HTTP/2 to mitigate pervasive
   monitoring attacks.  This mechanism not a replacement for "https"
   URIs; it is vulnerable to active attacks.

Status of This Memo

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   This Internet-Draft will expire on September 18, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Goals and Non-Goals . . . . . . . . . . . . . . . . . . .   2
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Using HTTP URIs over TLS  . . . . . . . . . . . . . . . . . .   3
     2.1.  Alternative Server Opt-In . . . . . . . . . . . . . . . .   4
     2.2.  Interaction with "https" URIs . . . . . . . . . . . . . .   5
     2.3.  The "http-opportunistic" well-known URI . . . . . . . . .   5
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     4.1.  Security Indicators . . . . . . . . . . . . . . . . . . .   6
     4.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Privacy Considerations  . . . . . . . . . . . . . . . . .   6
     4.4.  Confusion Regarding Request Scheme  . . . . . . . . . . .   7
     4.5.  Server Controls . . . . . . . . . . . . . . . . . . . . .   7
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   This document describes a use of HTTP Alternative Services [RFC7838]
   to decouple the URI scheme from the use and configuration of
   underlying encryption.  It allows an "http" URI to be accessed using
   HTTP/2 [RFC7230] and Transport Layer Security (TLS) [RFC5246] with
   Opportunistic Security [RFC7435].

   This document describes a usage model whereby sites can serve "http"
   URIs over TLS, thereby avoiding the problem of serving Mixed Content
   (described in [W3C.CR-mixed-content-20160802]) while still providing
   protection against passive attacks.

   Opportunistic Security does not provide the same guarantees as using
   TLS with "https" URIs, because it is vulnerable to active attacks,
   and does not change the security context of the connection.
   Normally, users will not be able to tell that it is in use (i.e.,
   there will be no "lock icon").

1.1.  Goals and Non-Goals

   The immediate goal is to make the use of HTTP more robust in the face
   of pervasive passive monitoring [RFC7258].

   A secondary (but significant) goal is to provide for ease of
   implementation, deployment and operation.  This mechanism is expected

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   to have a minimal impact upon performance, and require a trivial
   administrative effort to configure.

   Preventing active attacks (such as a Man-in-the-Middle) is a non-goal
   for this specification.  Furthermore, this specification is not
   intended to replace or offer an alternative to "https", since "https"
   both prevents active attacks and invokes a more stringent security
   model in most clients.

1.2.  Notational Conventions

   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.  Using HTTP URIs over TLS

   An origin server that supports the resolution of "http" URIs can
   indicate support for this specification by providing an alternative
   service advertisement [RFC7838] for a protocol identifier that uses
   TLS, such as "h2" [RFC7540].  Such a protocol MUST include an
   explicit indication of the scheme of the resource.  This excludes
   HTTP/1.1; HTTP/1.1 clients are forbidden from including the absolute
   form of a URI in requests to origin servers (see Section 5.3.1 of
   [RFC7230]).

   A client that receives such an advertisement MAY make future requests
   intended for the associated origin [RFC6454] to the identified
   service (as specified by [RFC7838]), provided that the alternative
   service opts in as described in Section 2.1.

   A client that places the importance of protection against passive
   attacks over performance might choose to withhold requests until an
   encrypted connection is available.  However, if such a connection
   cannot be successfully established, the client can resume its use of
   the cleartext connection.

   A client can also explicitly probe for an alternative service
   advertisement by sending a request that bears little or no sensitive
   information, such as one with the OPTIONS method.  Likewise, clients
   with existing alternative services information could make such a
   request before they expire, in order minimize the delays that might
   be incurred.

   Client certificates are not meaningful for URLs with the "http"
   scheme, and therefore clients creating new TLS connections to
   alternative services for the purposes of this specification MUST NOT
   present them.  A server that also provides "https" resources on the

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   same port can request a certificate during the TLS handshake, but it
   MUST NOT abort the handshake if the client does not provide one.

2.1.  Alternative Server Opt-In

   It is possible that the server might become confused about whether
   requests' URLs have a "http" or "https" scheme, for various reasons;
   see Section 4.4.  To ensure that the alternative service has opted
   into serving "http" URLs over TLS, clients are required to perform
   additional checks before directing "http" requests to it.

   Clients MUST NOT send "http" requests over a secured connection,
   unless the chosen alternative service presents a certificate that is
   valid for the origin as defined in [RFC2818].  Using an authenticated
   alternative service establishes "reasonable assurances" for the
   purposes of [RFC7838].  In addition to authenticating the server, the
   client MUST have obtained a valid http-opportunistic response for an
   origin (as per Section 2.3) using the authenticated connection.  An
   exception to the latter restriction is made for requests for the
   "http-opportunistic" well-known URI.

   For example, assuming the following request is made over a TLS
   connection that is successfully authenticated for those origins, the
   following request/response pair would allow requests for the origins
   "http://www.example.com" or "http://example.com" to be sent using a
   secured connection:

   HEADERS
     + END_STREAM
     + END_HEADERS
       :method = GET
       :scheme = http
       :authority = example.com
       :path = /.well-known/http-opportunistic

   HEADERS
       :status = 200
       content-type = application/json
   DATA
     + END_STREAM
   [ "http://www.example.com", "http://example.com" ]

   Though this document describes multiple origins, this is only for
   operational convenience.  Only a request made to an origin (over an
   authenticated connection) can be used to acquire this resource for
   that origin.  Thus in the example, the request to
   "http://example.com" cannot be assumed to also provide an http-
   opportunistic response for "http://www.example.com".

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2.2.  Interaction with "https" URIs

   Clients MUST NOT send "http" requests and "https" requests on the
   same connection.  Similarly, clients MUST NOT send "http" requests
   for multiple origins on the same connection.

2.3.  The "http-opportunistic" well-known URI

   This specification defines the "http-opportunistic" well-known URI
   [RFC5785].  A client is said to have a valid http-opportunistic
   response for a given origin when:

   o  The client has requested the well-known URI from the origin over
      an authenticated connection and a 200 (OK) response was provided,
      and

   o  That response is fresh [RFC7234] (potentially through revalidation
      [RFC7232]), and

   o  That response has the media type "application/json", and

   o  That response's payload, when parsed as JSON [RFC7159], contains
      an array as the root, and

   o  The array contains a string that is a case-insensitive character-
      for-character match for the origin in question, serialised into
      Unicode as per Section 6.1 of [RFC6454].

   A client MAY treat an "http-opportunistic" resource as invalid if
   values it contains are not strings.

   This document does not define semantics for "http-opportunistic"
   resources on an "https" origin, nor does it define semantics if the
   resource includes "https" origins.

   Allowing clients to cache the http-opportunistic resource means that
   all alternative services need to be able to respond to requests for
   "http" resources.  A client is permitted to use an alternative
   service without acquiring the http-opportunistic resource from that
   service.

   A client MUST NOT use any cached copies of an http-opportunistic
   resource that was acquired (or revalidated) over an unauthenticated
   connection.  To avoid potential errors, a client can request or
   revalidate the http-opportunistic resource before using any
   connection to an alternative service.

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   Clients that use cached http-opportunistic responses MUST ensure that
   their cache is cleared of any responses that were acquired over an
   unauthenticated connection.  Revalidating an unauthenticated response
   using an authenticated connection does not ensure the integrity of
   the response.

3.  IANA Considerations

   This specification registers a Well-Known URI [RFC5785]:

   o  URI Suffix: http-opportunistic

   o  Change Controller: IETF

   o  Specification Document(s): Section 2.3 of [this specification]

   o  Related Information:

4.  Security Considerations

4.1.  Security Indicators

   User Agents MUST NOT provide any special security indicators when an
   "http" resource is acquired using TLS.  In particular, indicators
   that might suggest the same level of security as "https" MUST NOT be
   used (e.g., a "lock device").

4.2.  Downgrade Attacks

   A downgrade attack against the negotiation for TLS is possible.

   For example, because the "Alt-Svc" header field [RFC7838] likely
   appears in an unauthenticated and unencrypted channel, it is subject
   to downgrade by network attackers.  In its simplest form, an attacker
   that wants the connection to remain in the clear need only strip the
   "Alt-Svc" header field from responses.

4.3.  Privacy Considerations

   Cached alternative services can be used to track clients over time;
   e.g., using a user-specific hostname.  Clearing the cache reduces the
   ability of servers to track clients; therefore clients MUST clear
   cached alternative service information when clearing other origin-
   based state (i.e., cookies).

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4.4.  Confusion Regarding Request Scheme

   HTTP implementations and applications sometimes use ambient signals
   to determine if a request is for an "https" resource; for example,
   they might look for TLS on the stack, or a server port number of 443.

   This might be due to expected limitations in the protocol (the most
   common HTTP/1.1 request form does not carry an explicit indication of
   the URI scheme and the resource might have been developed assuming
   HTTP/1.1), or it may be because how the server and application are
   implemented (often, they are two separate entities, with a variety of
   possible interfaces between them).

   Any security decisions based upon this information could be misled by
   the deployment of this specification, because it violates the
   assumption that the use of TLS (or port 443) means that the client is
   accessing a HTTPS URI, and operating in the security context implied
   by HTTPS.

   Therefore, server implementers and administrators need to carefully
   examine the use of such signals before deploying this specification.

4.5.  Server Controls

   This specification requires that a server send both an Alternative
   Service advertisement and host content in a well-known location to
   send HTTP requests over TLS.  Servers SHOULD take suitable measures
   to ensure that the content of the well-known resource remains under
   their control.  Likewise, because the Alt-Svc header field is used to
   describe policies across an entire origin, servers SHOULD NOT permit
   user content to set or modify the value of this header.

5.  References

5.1.  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>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

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   [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>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <http://www.rfc-editor.org/info/rfc5785>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <http://www.rfc-editor.org/info/rfc6454>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <http://www.rfc-editor.org/info/rfc7232>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <http://www.rfc-editor.org/info/rfc7234>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <http://www.rfc-editor.org/info/rfc7540>.

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <http://www.rfc-editor.org/info/rfc7838>.

5.2.  Informative References

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

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   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <http://www.rfc-editor.org/info/rfc7435>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <http://www.rfc-editor.org/info/rfc7469>.

   [W3C.CR-mixed-content-20160802]
              West, M., "Mixed Content", World Wide Web Consortium CR
              CR-mixed-content-20160802, August 2016,
              <https://www.w3.org/TR/2016/CR-mixed-content-20160802>.

Appendix A.  Acknowledgements

   Mike Bishop contributed significant text to this document.

   Thanks to Patrick McManus, Stefan Eissing, Eliot Lear, Stephen
   Farrell, Guy Podjarny, Stephen Ludin, Erik Nygren, Paul Hoffman, Adam
   Langley, Eric Rescorla, Julian Reschke, Kari Hurtta, and Richard
   Barnes for their feedback and suggestions.

Authors' Addresses

   Mark Nottingham

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/

   Martin Thomson
   Mozilla

   Email: martin.thomson@gmail.com

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