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Versions: 00

HTTP                                                             M. West
Internet-Draft                                                    Google
Intended status: Standards Track                       November 29, 2018
Expires: June 2, 2019


                        User Agent Client Hints
                     draft-west-ua-client-hints-00

Abstract

   This document defines a set of Client Hints that aim to provide
   developers with the ability to perform agent-based content
   negotiation when necessary, while avoiding the historical baggage and
   passive fingerprinting surface exposed by the venerable "User-Agent"
   header.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 2, 2019.

Copyright Notice

   Copyright (c) 2018 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
     1.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  User Agent Hints  . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The 'Sec-CH-Arch' Header Field  . . . . . . . . . . . . .   5
     2.2.  The 'Sec-CH-Model' Header Field . . . . . . . . . . . . .   5
     2.3.  The 'Sec-CH-Platform' Header Field  . . . . . . . . . . .   6
     2.4.  The 'Sec-CH-UA' Header Field  . . . . . . . . . . . . . .   6
     2.5.  Integration with Fetch  . . . . . . . . . . . . . . . . .   7
   3.  Security and Privacy Considerations . . . . . . . . . . . . .   8
     3.1.  Secure Transport  . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Delegation  . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Access Restrictions . . . . . . . . . . . . . . . . . . .   9
   4.  Implementation Considerations . . . . . . . . . . . . . . . .   9
     4.1.  The 'User-Agent' Header . . . . . . . . . . . . . . . . .   9
     4.2.  GREASE-like UA Strings  . . . . . . . . . . . . . . . . .   9
     4.3.  The 'Sec-CH-' prefix  . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  'Sec-CH-Arch' Header Field  . . . . . . . . . . . . . . .  10
     5.2.  'Sec-CH-Model' Header Field . . . . . . . . . . . . . . .  10
     5.3.  'Sec-CH-Platform' Header Field  . . . . . . . . . . . . .  11
     5.4.  'Sec-CH-UA' Header Field  . . . . . . . . . . . . . . . .  11
     5.5.  'User-Agent' Header Field . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .  13
     A.1.  draft-west-ua-client-hints-00 . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Today, user agents generally identify themselves to servers by
   sending a "User-Agent" HTTP request header field along with each
   request (defined in Section 5.5.3 of [RFC7231]).  Ideally, this
   header would give servers the ability to perform content negotiation,
   sending down exactly those bits that best represent the requested
   resource in a given user agent, optimizing both bandwidth and user
   experience.  In practice, however, this header's value exposes far
   more information about the user's device than seems appropriate as a
   default, on the one hand, and intentionally obscures the true user
   agent in order to bypass misguided server-side heuristics, on the
   other.

   For example, a recent version of Chrome on iOS identifies itself as:




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     User-Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 12_0 like Mac OS X)
                 AppleWebKit/605.1.15 (KHTML, like Gecko)
                 CriOS/69.0.3497.105 Mobile/15E148 Safari/605.1

   While a recent version of Edge identifies itself as:

  User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64)
              AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.2704.79
              Safari/537.36 Edge/18.014

   There's quite a bit of information packed into those strings (along
   with a fair number of lies).  Version numbers, platform details,
   model information, etc. are all broadcast along with every request,
   and form the basis for fingerprinting schemes of all sorts.
   Individual vendors have taken stabs at altering their user agent
   strings, and have run into a few categories of feedback from
   developers that have stymied historical approaches:

   1.  Brand and version information (e.g.  "Chrome 69") allows websites
       to work around known bugs in specific releases that aren't
       otherwise detectable.  For example, implementations of Content
       Security Policy have varied wildly between vendors, and it's
       difficult to know what policy to send in an HTTP response without
       knowing what browser is responsible for its parsing and
       execution.

   2.  Developers will often negotiate what content to send based on the
       user agent and platform.  Some application frameworks, for
       instance, will style an application on iOS differently from the
       same application on Android in order to match each platform's
       aesthetic and design patterns.

   3.  Similarly to #1, OS revisions and architecture can be responsible
       for specific bugs which can be worked around in website's code,
       and narrowly useful for things like selecting appropriate
       executables for download (32 vs 64 bit, ARM vs Intel, etc).

   4.  Sophisticated developers use model/make to tailor their sites to
       the capabilities of the device (e.g.  [FacebookYearClass]) and to
       pinpoint performance bugs and regressions which sometimes are
       specific to model/make.

   This document proposes a mechanism which might allow user agents to
   be a bit more aggressive about removing entropy from the "User-Agent"
   string generally by giving servers that really need some specific
   details about the client the ability to opt-into receiving them.  It
   introduces four new Client Hints ([I-D.ietf-httpbis-client-hints])
   that can provide the client's branding and version information, the



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   underlying operating system's branding and major version, as well as
   details about the underlying device.  Rather than broadcasting this
   data to everyone, all the time, user agents can make reasonable
   decisions about how to respond to given sites' requests for more
   granular data, reducing the passive fingerprinting surface area
   exposed to the network.

1.1.  Example

   A user navigates to "https://example.com/" for the first time.  Their
   user agent sends the following header along with the HTTP request:

     Sec-CH-UA: "Examplary Browser 73"

   The server is interested in rendering content consistent with the
   user's underlying platform, and asks for a little more information by
   sending an "Accept-CH" header (Section 2.2.1 of
   [I-D.ietf-httpbis-client-hints]) along with the initial response:

     Accept-CH: UA, Platform

   In response, the user agent includes more detailed version
   information, as well as information about the underlying platform in
   the next request:

     Sec-CH-UA: "Examplary Browser 73.3R8.2H.1"
     Sec-CH-Platform: "Windows 10"

1.2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  User Agent Hints

   The following sections define a number of HTTP request header fields
   that expose detail about a given user agent, which servers can opt-
   into receiving via the Client Hints infrastructure defined in
   [I-D.ietf-httpbis-client-hints].  The definitions below assume that
   each user agent has defined a number of properties for itself (all of
   which are strings):

   o  "brand" (for example: "cURL", "Edge", "The World's Best Web
      Browser")




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   o  "major version" (for example: "72", "3", or "28")

   o  "full version" (for example: "72.0.3245.12", "3.14159", or
      "297.70E04154A")

   o  "platform brand" (for example: "Windows NT", "iOS", or
      "AmazingOS")

   o  "platform version" (for example: "10", "12", or "17G")

   o  "platform architecture" (for example: "ARM64", or "ia32")

   o  "model" (for example: "", or "Pixel 2 XL")

   User agents SHOULD keep these strings short and to the point, but
   servers MUST accept arbitrary values for each, as they are all values
   constructed at the user agent's whim.

2.1.  The 'Sec-CH-Arch' Header Field

   The "Sec-CH-Platform" request header field gives a server information
   about the architecture of the platform on which a given user agent is
   executing.  It is a Structured Header
   ([I-D.ietf-httpbis-header-structure]) whose value MUST be a string
   ([I-D.ietf-httpbis-header-structure], Section 3.7).

   The header's ABNF is:

     Sec-CH-Arch = sh-string

   To generate a "Sec-CH-Arch" header value for a given request, user
   agents MUST:

   1.  If the request's client-hints set includes "Arch", then:

       1.  Let "value" be a Structured Header whose value is the user
           agent's "platform architecture".

       2.  Set a header in request's header list whose name is "Sec-CH-
           Arch", and whose value is "value".

2.2.  The 'Sec-CH-Model' Header Field

   The "Sec-CH-Model" request header field gives a server information
   about the device on which a given user agent is executing.  It is a
   Structured Header ([I-D.ietf-httpbis-header-structure]) whose value
   MUST be a string ([I-D.ietf-httpbis-header-structure], Section 3.7).




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   The header's ABNF is:

     Sec-CH-Model = sh-string

   To generate a "Sec-CH-Model" header value for a given request, user
   agents MUST:

   1.  If the request's client-hints set includes "Model", then:

       1.  Let "value" be a Structured Header whose value is the user
           agent's "model".

       2.  Set a header in request's header list whose name is "Sec-CH-
           Model", and whose value is "value".

2.3.  The 'Sec-CH-Platform' Header Field

   The "Sec-CH-Platform" request header field gives a server information
   about the platform on which a given user agent is executing.  It is a
   Structured Header ([I-D.ietf-httpbis-header-structure]) whose value
   MUST be a string ([I-D.ietf-httpbis-header-structure], Section 3.7).

   The header's ABNF is:

     Sec-CH-Platform = sh-string

   To generate a "Sec-CH-Platform" header value for a given request,
   user agents MUST:

   1.  If the request's client-hints set includes "Platform", then:

       1.  Let "value" be a Structured Header whose value is the
           concatenation of the user agent's "platform brand", a U+0020
           SPACE character, and the user agent's "platform version".

       2.  Set a header in request's header list whose name is "Sec-CH-
           Platform", and whose value is "value".

2.4.  The 'Sec-CH-UA' Header Field

   The "Sec-CH-UA" request header field gives a server information about
   a user agent's branding and version.  It is a Structured Header
   ([I-D.ietf-httpbis-header-structure]) whose value MUST be a list
   ([I-D.ietf-httpbis-header-structure], Section 3.2).  Each item in the
   list MUST be a string ([I-D.ietf-httpbis-header-structure],
   Section 3.7).

   The header's ABNF is:



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     Sec-CH-UA = sh-list

   Unlike most Client Hints, the "Sec-CH-UA" header will be sent with
   all requests, whether or not the server opted-into receiving the
   header via an "Accept-CH" header.  Prior to an opt-in, however, it
   will include only the user agent's branding information, and the
   major version number (both of which are fairly clearly sniffable by
   "examining the structure of other headers and by testing for the
   availability and semantics of the features introduced or modified
   between releases of a particular browser" [Janc2014]).

   To generate a "Sec-CH-UA" header value for a given request, user
   agents MUST:

   1.  Let "value" be a Structured Header whose value is a list
       ([I-D.ietf-httpbis-header-structure]).

   2.  If the request's client-hints set includes "UA", then add an item
       to "value" whose value is the concatenation of the user agent's
       "brand", a U+0020 SPACE character, and the user agent's "full
       version".

       Otherwise, add an item to "value" whose value is the
       concatenation of the user agent's "brand", a U+0020 SPACE
       character, and the user agent's "major version".

   3.  The user agent MAY execute the following steps:

       1.  Append additional items to "value" containing arbitrary brand
           and version combinations.

       2.  Randomize the order of the items in "value".

       Note: See Section 4.2 for more details on why these steps might
       be appropriate.

   4.  Set a header in request's header list whose name is "Sec-CH-UA",
       and whose value is "value".

2.5.  Integration with Fetch

   The Fetch specification should call into the following algorithm in
   place of the current Step 5.11 in its HTTP-network-or-cache fetch
   algorithm.

   To set the user agent metadata for a request ("r"), the user agent
   MUST execute the following steps:




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   1.  If request's header list does not contain "User-Agent", then the
       user agent MAY append "User-Agent"/default "User-Agent" value to
       "request"'s header list.

   2.  Set request's "Sec-CH-Arch" header, as described in Section 2.1.

   3.  Set request's "Sec-CH-Model" header, as described in Section 2.2.

   4.  Set request's "Sec-CH-Platform" header, as described in
       Section 2.3.

   5.  Set request's "Sec-CH-UA" header, as described in Section 2.4.

3.  Security and Privacy Considerations

3.1.  Secure Transport

   Client Hints will not be delivered to non-secure endpoints (see the
   secure transport requirements in Section 2.2.1 of
   [I-D.ietf-httpbis-client-hints]).  This means that user agent
   information will not be leaked over plaintext channels, reducing the
   opportunity for network attackers to build a profile of a given
   agent's behavior over time.

3.2.  Delegation

   Client Hints will be delegated from top-level pages via Feature
   Policy (once a few patches against Fetch and Client Hints and Feature
   Policy land.  This reduces the likelihood that user agent information
   will be delivered along with subresource requests, which reduces the
   potential for passive fingerprinting.

   o  Fetch integration of Accept-CH opt-in:
      https://github.com/whatwg/fetch/issues/773

   o  HTML integration of Accept-CH-Lifetime and the ACHL cache:
      https://github.com/whatwg/html/issues/3774

   o  Adding new CH features to the CH list in Fetch:
      https://github.com/whatwg/fetch/issues/725

   o  Other PRs for adding the Feature Policy 3rd party opt-in:
      https://github.com/whatwg/fetch/issues/811 and
      https://github.com/wicg/feature-policy/issues/220







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3.3.  Access Restrictions

   The information in the Client Hints defined above reveals quite a bit
   of information about the user agent and the platform/device upon
   which it runs.  User agents ought to exercise judgement before
   granting access to this information, and MAY impose restrictions
   above and beyond the secure transport and delegation requirements
   noted above.  For instance, user agents could choose to reveal
   "platform architecture" only on requests it intends to download,
   giving the server the opportunity to serve the right binary.
   Likewise, they could offer users control over the values revealed to
   servers, or gate access on explicit user interaction via a permission
   prompt or via a settings interface.

4.  Implementation Considerations

4.1.  The 'User-Agent' Header

   User agents SHOULD deprecate the "User-Agent" header in favor of the
   Client Hints model described in this document.  The header, however,
   is likely to be impossible to remove entirely in the near-term, as
   existing sites' content negotiation code will continue to require its
   presence (see [Rossi2015] for a recent example of a new browser's
   struggles in this area).

   One approach which might be advisable could be for each user agent to
   lock the value of its "User-Agent" header, ensuring backwards
   compatibility by maintaining the crufty declarations of "like Gecko"
   and "AppleWebKit/537.36" on into eternity.  This can ratchet over
   time, first freezing the version number, then shifting platform and
   model information to something reasonably generic in order to reduce
   the fingerprint the header provides.

4.2.  GREASE-like UA Strings

   History has shown us that there are real incentives for user agents
   to lie about their branding in order to thread the needle of sites'
   sniffing scripts.  While I'm optimistic that we can reset
   expectations around sniffing by freezing the thing that's sniffed-
   upon today, and creating a sane set of options for developers, it's
   likely that this is hopelessly naive.  It's reasonable to ponder what
   we should do to encourage sniffing in the right way, if we believe
   it's going to happen one way or another.

   User agents may choose to model "UA" as a set, rather than a single
   entry.  This could encourage standardized processing of the "UA"
   string by Randomly including additional, intentionally incorrect,
   comma-separated entries with arbitrary ordering (similar conceptually



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   to [I-D.ietf-tls-grease]) could encourage standardized processing if
   the "UA" string by servers, and reduce the chance that we ossify on a
   few required strings.  For example, Chrome 73's "Sec-CH-UA" header
   might be ""Chrome 73", "NotBrowser 12"", or ""BrowsingIsFun Version
   12b", "Chrome 73"", or something completely different.

4.3.  The 'Sec-CH-' prefix

   Based on some discussion in https://github.com/w3ctag/design-reviews/
   issues/320, it seems reasonable to forbid access to these headers
   from JavaScript, and demarcate them as browser-controlled client
   hints so they can be documented and included in requests without
   triggering CORS preflights.  A "Sec-CH-" prefix seems like a viable
   approach, but this bit might shift as the broader Client Hints
   discussions above coalesce into something more solid that lands in
   specs.

5.  IANA Considerations

   This document intends to define the "Sec-CH-Arch", "Sec-CH-Model",
   "Sec-CH-Platform", and "Sec-CH-UA" HTTP request header fields, and
   register them in the permanent message header field registry
   ([RFC3864]).

   It also intends to deprecate the "User-Agent" header field.

5.1.  'Sec-CH-Arch' Header Field

   Header field name:  Sec-CH-Arch

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF

   Specification document:  this specification (Section 2.1)

5.2.  'Sec-CH-Model' Header Field

   Header field name:  Sec-CH-Model

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF




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   Specification document:  this specification (Section 2.4)

5.3.  'Sec-CH-Platform' Header Field

   Header field name:  Sec-CH-Platform

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF

   Specification document:  this specification (Section 2.3)

5.4.  'Sec-CH-UA' Header Field

   Header field name:  Sec-CH-UA

   Applicable protocol:  http

   Status:  standard

   Author/Change controller:  IETF

   Specification document:  this specification (Section 2.4)

5.5.  'User-Agent' Header Field

   Header field name:  User-Agent

   Applicable protocol:  http

   Status:  deprecated

   Author/Change controller:  IETF

   Specification document:  this specification (Section 4.1), and
      Section 5.5.3 of [RFC7231]

6.  References

6.1.  Normative References

   [I-D.ietf-httpbis-client-hints]
              Grigorik, I., "HTTP Client Hints", draft-ietf-httpbis-
              client-hints-06 (work in progress), July 2018.





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   [I-D.ietf-httpbis-header-structure]
              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-08 (work in progress),
              October 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004, <https://www.rfc-
              editor.org/info/rfc3864>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014, <https://www.rfc-
              editor.org/info/rfc7231>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

6.2.  Informative References

   [FacebookYearClass]
              Marra, C. and D. Weaver, "Year class: A classification
              system for Android", November 2014,
              <https://code.fb.com/android/year-class-a-classification-
              system-for-android/>.

   [I-D.ietf-tls-grease]
              Benjamin, D., "Applying GREASE to TLS Extensibility",
              draft-ietf-tls-grease-01 (work in progress), June 2018.

   [Janc2014]
              Zalewski, M. and A. Janc, "Technical analysis of client
              identification mechanisms", September 2014,
              <https://dev.chromium.org/Home/chromium-security/client-
              identification-mechanisms#TOC-Browser-level-fingerprints>.

   [Rossi2015]
              Rossi, J., "The Microsoft Edge Rendering Engine that makes
              the Web just work", May 2015,
              <https://channel9.msdn.com/Events/WebPlatformSummit/2015/
              The-Microsoft-Edge-Rendering-Engine-that-makes-the-Web-
              just-work#time=9m45s>.



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Appendix A.  Changes

A.1.  draft-west-ua-client-hints-00

   o  This specification sprang, fully-formed, from the head of Zeus.

Author's Address

   Mike West
   Google

   Email: mkwst@google.com







































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