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The "safe" HTTP Preference

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8674.
Author Mark Nottingham
Last updated 2019-04-30 (Latest revision 2019-04-17)
RFC stream Independent Submission
IETF conflict review conflict-review-nottingham-safe-hint, conflict-review-nottingham-safe-hint, conflict-review-nottingham-safe-hint, conflict-review-nottingham-safe-hint, conflict-review-nottingham-safe-hint, conflict-review-nottingham-safe-hint, conflict-review-nottingham-safe-hint, conflict-review-nottingham-safe-hint, conflict-review-nottingham-safe-hint
Stream ISE state In IESG Review
Consensus boilerplate Yes
Document shepherd Eliot Lear
Shepherd write-up Show Last changed 2019-04-18
IESG IESG state Became RFC 8674 (Informational)
Telechat date (None)
Responsible AD Barry Leiba
Send notices to Adrian Farrel <>
IANA IANA review state IANA OK - Actions Needed
Network Working Group                                      M. Nottingham
Internet-Draft                                            April 17, 2019
Intended status: Informational
Expires: October 19, 2019

                       The "safe" HTTP Preference


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

   Support for this preference by clients and servers is optional.

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

   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 October 19, 2019.

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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The "safe" Preference . . . . . . . . . . . . . . . . . . . .   3
   3.  Implementation Status . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Sending "safe" from Web Browsers . . . . . . . . . .   7
   Appendix C.  Supporting "safe" on Web Sites . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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, 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 assure 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
   a HTTP Preference [RFC7240].

   Note that this specification does not define what content might be
   considered objectionable, and so the concept of "safe" is also 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

   That said, the intent of "safe" 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) content that is unsuitable for minors.  The
   "safe" preference is not intended to be used for other purposes.

   Furthermore, sending "safe" does not guarantee that the Web site will
   use it, nor that it will apply a concept of "objectionable" that is

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   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

   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 to
   the effect "If your site has a 'safe' setting, this user is hereby
   opting into that, according to your definition of the term."

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 to not respond with content which
   is designated as objectionable, according to the origin server's
   definition of the concept.

   For example, a request that includes the "safe" preference:

   GET /foo.html HTTP/1.1
   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

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   scheme (see Section 4).  See Appendix B for more information about
   configuring the set of resources "safe" is sent to.

   Safe 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

   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 [RFC7234] Section 4.1 for more information the interaction
   between Vary and Web caching.

   See Appendix C for additional advice specific to Web servers wishing
   to use "safe".

3.  Implementation Status

   _Note to RFC Editor: Please remove this section before publication._

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft.  Please note that the listing of any individual
   implementation here does not imply endorsement by the IETF.

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   Furthermore, no effort has been spent to verify the information
   presented here that was supplied by IETF contributors.  This is not
   intended as, and must not be construed to be, a catalog of available
   implementations or their features.  Readers are advised to note that
   other implementations may exist.

   o  Microsoft Internet Explorer - see

   o  Microsoft Bing - see

   o  Mozilla Firefox - see

   o  Cisco - see

4.  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 limited information about the user, which may be
   of small assistance in "fingerprinting" the user by sites.
   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 "safe" in requests does not assure that all
   content will actually be safe; it is only when servers elect to honor
   it that content might be "safe".

   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 assure 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.

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5.  IANA Considerations

   This specification registers the following entry in the "HTTP
   Preferences" registry [RFC7240]:

   o  Preference: safe

   o  Value: (no value)

   o  Description: Indicates that "safe" / "unobjectionable" content is

   o  Reference: (this document)

   o  Notes:

6.  References

6.1.  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,

   [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,

   [RFC7240]  Snell, J., "Prefer Header for HTTP", RFC 7240,
              DOI 10.17487/RFC7240, June 2014,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

6.2.  Informative References

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,

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Appendix A.  Acknowledgements

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

Appendix B.  Sending "safe" 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" - that
   is, prevent modification without administrative access, or a

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

Appendix C.  Supporting "safe" 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

   When honoring the safe preference, it is important that it not be
   possible to disable it through the Web site's interface, since "safe"
   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

   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 through other

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   means, especially if the preference is generated based upon Operating
   System configuration.

   Sites might offer different levels of "safeness" 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 the user expresses 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 "safe" 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).

Author's Address

   Mark Nottingham


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