The "safe" HTTP Preference
mnot@mnot.net
https://www.mnot.net/
General
safe
preference
child-protection
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.
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 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 .
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.
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
when, and only when, they appear in all capitals,
as shown here.
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:
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 ). See 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 ) to fine-tune.
A response corresponding to the request above might have headers that look like this:
Here, the Preference-Applied response header 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 for
more information about the interaction between the Vary header field and
web caching.
See for additional advice
specific to web servers wishing to use the safe preference.
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.
IANA Considerations
Per this specification, IANA has registered the following entry in
the "HTTP Preferences" registry :
- Preference: safe
- Description: Indicates that safe (i.e., unobjectionable) content is preferred.
- Reference: RFC 8674
References
Normative References
Informative References
Sending the "safe" Preference from Web Browsers
As discussed in , 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.
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 , 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.