The "safe" HTTP Preference
RFC 8674

Document Type RFC - Informational (December 2019; No errata)
Author Mark Nottingham 
Last updated 2019-12-04
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Independent Submission                                     M. Nottingham
Request for Comments: 8674                                 December 2019
Category: Informational                                                 
ISSN: 2070-1721

                       The "safe" HTTP Preference

Abstract

   This specification defines a preference for HTTP requests that
   expresses a desire to avoid objectionable content, according to the
   definition of that term by the origin server.

   This specification does not define a precise semantic for "safe".
   Rather, the term is interpreted by the server and within the scope of
   each web site that chooses to act upon this information.

   Support for this preference by clients and servers is optional.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates 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
   https://www.rfc-editor.org/info/rfc8674.

Copyright Notice

   Copyright (c) 2019 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
   (https://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.

Table of Contents

   1.  Introduction
     1.1.  Notational Conventions
   2.  The "safe" Preference
   3.  Security Considerations
   4.  IANA Considerations
   5.  References
     5.1.  Normative References
     5.2.  Informative References
   Appendix A.  Sending the "safe" Preference from Web Browsers
   Appendix B.  Supporting the "safe" Preference on Web Sites
   Acknowledgements
   Author's Address

1.  Introduction

   Many web sites have a "safe" mode to assist those who don't want to
   be exposed (or have their children exposed) to content to which they
   might object.

   However, that goal is often difficult to achieve because of the need
   to go to every web site that might be used and navigate to the
   appropriate page (possibly creating an account along the way) to get
   a cookie [RFC6265] set in the browser, for each browser on every
   device used.

   A more manageable approach is for the browser to proactively indicate
   a preference for safe content.  A user agent that supports doing so
   (whether it be an individual browser or through an operating system
   HTTP library) need only be configured once to ensure that the
   preference is advertised to a set of sites, or even all sites.

   This specification defines how to declare this desire in requests as
   an HTTP Preference [RFC7240].

   Note that this specification does not define what content might be
   considered objectionable, so the concept of "safe" is not precisely
   defined.  Rather, the term is interpreted by the server and within
   the scope of each web site that chooses to act upon this information.

   That said, the intent is to allow end users (or those acting on their
   behalf) to express a desire to avoid content that is considered
   objectionable within the cultural context of that site; usually (but
   not always), the objectionable content is content unsuitable for
   minors.  The safe preference is not intended to be used for other
   purposes.

   Furthermore, sending the preference does not guarantee that the web
   site will use it or that it will apply a concept of "objectionable"
   that is consistent with the requester's views.  As such, its effect
   can be described as "best effort" and not to be relied upon.  In
   other words, sending the preference is no more reliable than going to
   each web site and manually selecting a safe mode, but it is
   considerably easier.

   It is also important to note that the safe preference is not a
   reliable indicator that the end user is a child; other users might
   have a desire for unobjectionable content, and some children might
   browse without the preference being set.

   Note also that the cultural context applies to the hosting location
   of a site, the content provider, and the source of the content.  It
   cannot be guaranteed that a user agent and origin server will have
   the same view of the concept of what is objectionable.

   Simply put, it is a statement by (or on behalf of) the end user
   indicating that "if your site has a safe setting, this user is hereby
   opting into that, according to your definition of the term."

   The mechanism described in this document does not have IETF consensus
   and is not a standard.  It is a widely deployed approach that has
   turned out to be useful and is presented here so that server and
   browser implementations can have a common understanding of how it
   operates.

   This mechanism was presented for publication as an IETF Proposed
   Standard but was not approved for publication by the IESG because of
   concerns that included the vagueness of the meaning of "safe", the
   ability of a proxy to insert the hint outside of a user's control,
   the fact that there was no way to know whether the hint was or was
   not applied to the response returned by the server, and the
   possibility that the use of this preference may incentivize increased
   censorship and/or targeting of minors.

   The specification was updated to address those concerns, but the IESG
   did not approve progressing this document as an IETF Proposed
   Standard.  As a result, it has been published in the Independent
   Stream.

1.1.  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.  The "safe" Preference

   When present in a request, the safe preference indicates that the
   user prefers that the origin server not respond with content that is
   designated as objectionable, according to the origin server's
   definition of the concept.

   For example, this is a request that includes the safe preference:

   GET /foo.html HTTP/1.1
   Host: www.example.org
   User-Agent: ExampleBrowser/1.0
   Prefer: safe

   Typically, user agents that emit the safe preference will include it
   in all requests with the "https" URI scheme, although some might
   expose finer-grained controls over when it is sent; this ensures that
   the preference is available to the applicable resources.  User agents
   MUST NOT emit the safe preference on requests with the "http" URI
   scheme (see Section 3).  See Appendix A for more information about
   configuring the set of resources the safe preference is sent to.

   The safe preference MAY be implemented in common HTTP libraries
   (e.g., an operating system might choose to insert the preference in
   requests based upon system-wide configuration).

   Origin servers that utilize the safe preference ought to document
   that they do so, along with the criteria that they use to denote
   objectionable content.  If a server has more fine-grained degrees of
   safety, it SHOULD select a reasonable default to use and document
   that; it MAY use additional mechanisms (e.g., cookies [RFC6265]) to
   fine-tune.

   A response corresponding to the request above might have headers that
   look like this:

   HTTP/1.1 200 OK
   Transfer-Encoding: chunked
   Content-Type: text/html
   Preference-Applied: safe
   Server: ExampleServer/2.0
   Vary: Prefer

   Here, the Preference-Applied response header [RFC7240] indicates that
   the site has applied the preference.  Servers are not required to
   send Preference-Applied (even when they have applied the preference)
   but are encouraged to where possible.

   Note that the Vary response header needs to be sent if the response
   is cacheable and might change depending on the value of the Prefer
   header.  This is not only true for those responses that are safe but
   also the default unsafe response.

   See Section 4.1 of [RFC7234] for more information about the
   interaction between the Vary header field and web caching.

   See Appendix B for additional advice specific to web servers wishing
   to use the safe preference.

3.  Security Considerations

   The safe preference is not a secure mechanism; it can be inserted or
   removed by intermediaries with access to the request stream (e.g.,
   for "http" URLs).  Therefore, it is prohibited from being included in
   requests with the "http" scheme.

   Its presence reveals information about the user, which may be of
   assistance in fingerprinting the user by sites and other entities in
   the network.  This information provides insight into the preferences
   of the user and might be used to make assumptions about user; thus,
   it could be used to identify categories of users for purposes such as
   targeting (including advertising and identification of minors).
   Therefore, user agents SHOULD NOT include it in requests when the
   user has expressed a desire to avoid such attacks (e.g., some forms
   of private mode browsing).

   By its nature, including the safe preference in requests does not
   ensure that all content will actually be safe; content is safe only
   when servers elect to honor it.

   Even then, a malicious server might adapt content so that it is even
   less safe (by some definition of the word).  As such, this mechanism
   on its own is not enough to ensure that only safe content is seen;
   those who wish to ensure that will need to combine its use with other
   techniques (e.g., content filtering).

   Furthermore, the server and user may have differing ideas regarding
   the semantics of "safe".  As such, the safety of the user's
   experience when browsing from site to site, as well as over time,
   might (and probably will) change.

4.  IANA Considerations

   Per this specification, IANA has registered the following entry in
   the "HTTP Preferences" registry [RFC7240]:

   *  Preference: safe

   *  Description: Indicates that safe (i.e., unobjectionable) content
      is preferred.

   *  Reference: RFC 8674

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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC7240]  Snell, J., "Prefer Header for HTTP", RFC 7240,
              DOI 10.17487/RFC7240, June 2014,
              <https://www.rfc-editor.org/info/rfc7240>.

   [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>.

5.2.  Informative References

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

Appendix A.  Sending the "safe" Preference from Web Browsers

   As discussed in Section 2, there are many possible ways for the safe
   preference to be generated.  One possibility is for a web browser to
   allow its users to configure the preference to be sent.

   When doing so, it is important not to misrepresent the preference as
   binding to web sites.  For example, an appropriate setting might be a
   checkbox with wording such as:

      [] Request safe content from web sites

   along with further information available upon request.

   Browsers might also allow the safe preference to be locked to prevent
   modification without administrative access or a passcode.

   Note that this specification does not require browsers to send the
   safe preference on all requests, although that is one possible
   implementation; alternate implementation strategies include
   blacklists and whitelists.

Appendix B.  Supporting the "safe" Preference on Web Sites

   Web sites that allow configuration of a safe mode (for example, using
   a cookie) can add support for the safe preference incrementally;
   since the preference will not be supported by all clients
   immediately, it is necessary to have another way to configure it.

   When honoring the safe preference, it is important that it not be
   possible to disable it through the web site's interface, since the
   safe preference may be configured and locked down by the browser or
   computer's administrator (e.g., a parent).  If the site has such a
   means of configuration (e.g., stored user preferences) and the safe
   preference is received in a request, the "safer" interpretation ought
   to be used.

   The appropriate level of safety is a site-specific decision.  When
   selecting it, sites ought to bear in mind that disabling the
   preference might be considerably more onerous than using other means,
   especially if the preference is generated based upon the operating
   system configuration.

   Sites might offer different levels of safety through web
   configuration; they will need to either inform their users of what
   level the safe hint corresponds to or provide them with some means of
   adjusting it.

   If users express a wish to disable safe mode, the site can remind
   them that the safe preference is being sent and ask them to consult
   their administrator (since the safe preference might be set by a
   locked-down operating system configuration).

   As explained in Section 2, responses that change based upon the
   presence of the safe preference need to either carry the "Vary:
   Prefer" response header field or be uncacheable by shared caches
   (e.g., with a "Cache-Control: private" response header field).  This
   is to avoid an unsafe cached response being served to a client that
   prefers safe content (or vice versa).

Acknowledgements

   Thanks to Alissa Cooper, Ilya Grigorik, Emma Llanso, Jeff Hughes,
   Lorrie Cranor, Doug Turner, and Dave Crocker for their comments.

Author's Address

   Mark Nottingham

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