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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/