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rfc:rfc8164

Internet Engineering Task Force (IETF) M. Nottingham Request for Comments: 8164 Category: Experimental M. Thomson ISSN: 2070-1721 Mozilla

                                                              May 2017
                 Opportunistic Security for HTTP/2

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

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  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).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 7841.
 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/rfc8164.

Copyright Notice

 Copyright (c) 2017 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.

Nottingham & Thomson Experimental [Page 1] RFC 8164 Opportunistic HTTP/2 Security May 2017

Table of Contents

 1. Introduction ....................................................2
    1.1. Goals and Non-goals ........................................3
    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 .........................................7
    4.1. Security Indicators ........................................7
    4.2. Downgrade Attacks ..........................................7
    4.3. Privacy Considerations .....................................7
    4.4. Confusion regarding Request Scheme .........................7
    4.5. Server Controls ............................................8
 5. References ......................................................8
    5.1. Normative References .......................................8
    5.2. Informative References .....................................9
 Acknowledgements ...................................................9
 Authors' Addresses ................................................10

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 [RFC7230] to be
 accessed using HTTP/2 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").

Nottingham & Thomson Experimental [Page 2] RFC 8164 Opportunistic HTTP/2 Security May 2017

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 to have a minimal impact upon performance and require
 trivial administrative effort to configure.
 Preventing active attacks (such as man-in-the-middle attacks) 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.

Nottingham & Thomson Experimental [Page 3] RFC 8164 Opportunistic HTTP/2 Security May 2017

 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; 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
 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

 For various reasons, it is possible that the server might become
 confused about whether requests' URLs have an "http" or "https"
 scheme (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:

Nottingham & Thomson Experimental [Page 4] RFC 8164 Opportunistic HTTP/2 Security May 2017

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

2.2. Interaction with "https" URIs

 Clients MUST NOT send "http" 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,
 o  That response is fresh [RFC7234] (potentially through revalidation
    [RFC7232]),
 o  That response has the media type "application/json",
 o  That response's payload, when parsed as JSON [RFC7159], contains
    an array as the root, and

Nottingham & Thomson Experimental [Page 5] RFC 8164 Opportunistic HTTP/2 Security May 2017

 o  The array contains a string that is a case-insensitive, character-
    for-character match for the origin in question, serialized 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.
 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 the following well-known URI [RFC5785]:
 o  URI Suffix: http-opportunistic
 o  Change Controller: IETF
 o  Specification Document(s): Section 2.3 of RFC 8164
 o  Related Information:

Nottingham & Thomson Experimental [Page 6] RFC 8164 Opportunistic HTTP/2 Security May 2017

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).

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 of 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 an 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.

Nottingham & Thomson Experimental [Page 7] RFC 8164 Opportunistic HTTP/2 Security May 2017

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>.
 [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>.

Nottingham & Thomson Experimental [Page 8] RFC 8164 Opportunistic HTTP/2 Security May 2017

 [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>.
 [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>.
 [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>.

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.

Nottingham & Thomson Experimental [Page 9] RFC 8164 Opportunistic HTTP/2 Security May 2017

Authors' Addresses

 Mark Nottingham
 Email: mnot@mnot.net
 URI:   https://www.mnot.net/
 Martin Thomson
 Mozilla
 Email: martin.thomson@gmail.com

Nottingham & Thomson Experimental [Page 10]

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