HTTP Client Hints
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ilya@igvita.com
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Google
yoav@yoav.ws
https://blog.yoav.ws/
Applications and Real-Time
HTTP
Content Negotiation
HTTP defines proactive content negotiation to allow servers to select
the appropriate response for a given request, based upon the user
agent's characteristics, as expressed in request headers. In practice,
user agents are often unwilling to send those request headers, because
it is not clear whether they will be used, and sending them impacts both
performance and privacy.
This document defines an Accept-CH response header that servers can
use to advertise their use of request headers for proactive content
negotiation, along with a set of guidelines for the creation of such
headers, colloquially known as "Client Hints."
Introduction
There are thousands of different devices accessing the web, each with
different device capabilities and preference information. These device
capabilities include hardware and software characteristics, as well as
dynamic user and user agent preferences. Historically, applications that
wanted the server to optimize content delivery and user experience based
on such capabilities had to rely on passive identification (e.g., by
matching the User-Agent header field () against an established database of
user agent signatures), use HTTP cookies and URL parameters, or use some combination of these
and similar mechanisms to enable ad hoc content negotiation.
Such techniques are expensive to set up and maintain and are not
portable across both applications and servers. They also make it hard
for both user agent and server to understand which data are required and
are in use during the negotiation:
- User agent detection cannot reliably identify all static
variables, cannot infer dynamic user agent preferences, requires an
external device database, is not cache friendly, and is reliant on a
passive fingerprinting surface.
- Cookie-based approaches are not portable across applications and
servers, impose additional client-side latency by requiring JavaScript
execution, and are not cache friendly.
- URL parameters, similar to cookie-based approaches, suffer from
lack of portability and are hard to deploy due to a requirement to
encode content negotiation data inside of the URL of each
resource.
Proactive content negotiation () offers an alternative approach;
user agents use specified, well-defined request headers to advertise
their capabilities and characteristics, so that servers can select (or
formulate) an appropriate response based on those request headers (or on
other, implicit characteristics).
However, traditional proactive content negotiation techniques often
mean that user agents send these request headers prolifically. This
causes performance concerns (because it creates "bloat" in requests), as
well as privacy issues; passively providing such information allows
servers to silently fingerprint the user.
This document defines Client Hints, a framework that enables servers
to opt-in to specific proactive content negotiation features, adapting
their content accordingly, as well as guidelines for content negotiation
mechanisms that use the framework. This document also defines a new
response header, Accept-CH, that allows an origin server to explicitly
ask that user agents send these headers in requests.
Client Hints mitigate performance concerns by assuring that user
agents will only send the request headers when they're actually going to
be used, and they mitigate privacy concerns of passive fingerprinting by requiring
explicit opt-in and disclosure of required headers by the server through
the use of the Accept-CH response header, turning passive fingerprinting
vectors into active ones.
The document does not define specific usages of Client Hints. Such
usages need to be defined in their respective specifications.
One example of such usage is the User-Agent Client Hints .
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.
This document uses the Augmented Backus-Naur Form (ABNF) notation
of .
Client Hints Request Header Fields
A Client Hints request header field is an HTTP header field that is
used by HTTP user agents to indicate data that can be used by the server
to select an appropriate response. Each one conveys user-agent
preferences that the server can use to adapt and optimize the
response.
Sending Client Hints
User agents choose what Client Hints to send in a request based on
their default settings, user configuration, and server preferences
expressed in Accept-CH. The user agent and server can use an
opt-in mechanism outlined below to negotiate which header fields need
to be sent to allow for efficient content adaption, and they can optionally use
additional mechanisms (e.g., as outlined in ) to negotiate
delegation policies that control access of third parties to those same
header fields. User agents SHOULD require an opt-in to send any hints
that are not considered low-entropy. See the low-entropy hint table
at for
examples of hints that expose low amounts of entropy.
Implementers need to be aware of the fingerprinting implications
when implementing support for Client Hints and follow the
considerations outlined in the Security Considerations section of this
document (see ).
Server Processing of Client Hints
When presented with a request that contains one or more Client Hints
header fields, servers can optimize the response based upon the
information in them. When doing so, and if the resource is cacheable,
the server MUST also generate a Vary response header field
() to indicate which
hints can affect the selected response and whether the selected
response is appropriate for a later request.
Servers MUST ignore hints they do not understand nor support. There
is no mechanism for servers to indicate to user agents that hints were
ignored.
Furthermore, the server can generate additional response header
fields (as specified by the hint or hints in use) that convey related
values to aid client processing.
Advertising Server Support
Servers can advertise support for Client Hints using the mechanism described below.
The Accept-CH Response Header Field
The Accept-CH response header field indicates server support for
the hints indicated in its value. Servers wishing to receive user agent
information through Client Hints SHOULD add the Accept-CH response
header to their responses as early as possible.
Accept-CH is a Structured Header . Its
value MUST be an sf-list () whose members are Tokens (). Its ABNF is:
For example:
When a user agent receives an HTTP response containing
Accept-CH, it indicates that the origin opts-in to receive
the indicated request header fields for subsequent same-origin
requests. The opt-in MUST be ignored if delivered over non-secure
transport (using a scheme different from HTTPS). It SHOULD be
persisted and bound to the origin to enable delivery of Client Hints
on subsequent requests to the server's origin, for the duration of the
user's session (as defined by the user agent). An opt-in overrides
previous persisted opt-in values and SHOULD be persisted in its
stead.
Based on the Accept-CH example above, which is received in response
to a user agent navigating to "https://site.example", and delivered
over a secure transport, persisted Accept-CH preferences will be bound
to "https://site.example". It will then use it for navigations to
for example, "https://site.example/foobar.html", but not to, for example,
"https://foobar.site.example/". It will similarly use the preference
for any same-origin resource requests (e.g., to
"https://site.example/image.jpg") initiated by the page constructed
from the navigation's response, but not to cross-origin resource
requests (e.g., "https://thirdparty.example/resource.js"). This
preference will not extend to resource requests initiated to
"https://site.example" from other origins (e.g., from navigations to
"https://other.example/").
Interaction with Caches
When selecting a response based on one or more Client Hints, and if
the resource is cacheable, the server needs to generate a Vary
response header field to
indicate which hints can affect the selected response and whether the
selected response is appropriate for a later request.
The above example indicates that the cache key needs to include the
Sec-CH-Example header field.
The above example indicates that the cache key needs to include the
Sec-CH-Example and Sec-CH-Example-2 header fields.
Security Considerations
Information Exposure
Request header fields used in features relying on this document
expose information about the user's environment to enable
privacy-preserving proactive content negotiation and avoid exposing
passive fingerprinting vectors. However, implementers need to bear in
mind that in the worst case, uncontrolled and unmonitored active
fingerprinting is not better than passive fingerprinting. In order to
provide user privacy benefits, user agents need to apply further
policies that prevent abuse of the information exposed by features
using Client Hints.
The information exposed by features might reveal new information
about the user, and implementers ought to consider the following
considerations, recommendations, and best practices.
The underlying assumption is that exposing information about the
user as a request header is equivalent (from a security perspective)
to exposing this information by other means. (For example, if the
request's origin can access that information using JavaScript APIs
and transmit it to its servers.)
Because Client Hints is an explicit opt-in mechanism, it means
that servers wanting access to information about the user's
environment need to actively ask for it, enabling clients and privacy
researchers to keep track of which origins collect that data, and
potentially act upon it.
The header-based opt-in means that removal of passive fingerprinting vectors
is possible. As an example, the user agent can reduce the information
exposed by the User-Agent string, while enabling active access to that
information through User-Agent Client Hints .
Otherwise, the user agent can expose information already available through
script (e.g., the Save-Data Client Hints ),
without increasing the passive fingerprinting surface. User agents supporting
Client Hints features which send certain information to opted-in servers
SHOULD avoid sending the equivalent information passively.
Therefore, features relying on this document to define Client Hint
headers MUST NOT provide new information that is otherwise not made
available to the application by the user agent, such as existing
request headers, HTML, CSS, or JavaScript.
Such features need to take into account the following aspects of
the exposed information:
- Entropy:
- Exposing highly granular data can be used to help
identify users across multiple requests to different
origins. Reducing the set of header field values that can be
expressed, or restricting them to an enumerated range where the
advertised value is close to but is not an exact representation of
the current value, can improve privacy and reduce risk of
linkability by ensuring that the same value is sent by multiple
users.
- Sensitivity:
- The feature SHOULD NOT expose user-sensitive
information. To that end, information available to the application,
but gated behind specific user actions (e.g., a permission prompt or
user activation), SHOULD NOT be exposed as a Client Hint.
- Change over time:
- The feature SHOULD NOT expose user
information that changes over time, unless the state change itself
is also exposed (e.g., through JavaScript callbacks).
Different features will be positioned in different points in the
space between low-entropy, non-sensitive, and static information (e.g.,
user agent information) and high-entropy, sensitive, and dynamic
information (e.g., geolocation). User agents need to consider the
value provided by a particular feature vs. these considerations and
may wish to have different policies regarding that tradeoff on a
per-feature or other fine-grained basis.
Implementers ought to consider both user- and server-controlled
mechanisms and policies to control which Client Hints header fields
are advertised:
- Implementers SHOULD restrict delivery of some or all Client
Hints header fields to the opt-in origin only, unless the opt-in
origin has explicitly delegated permission to another origin to
request Client Hints header fields.
- Implementers that consider providing user-choice mechanisms that
allow users to balance privacy concerns against bandwidth
limitations need to also consider that explaining the
privacy implications involved to users, such as the risks of passive
fingerprinting, may be challenging or even impractical.
- Implementations specific to certain use cases or threat models
MAY avoid transmitting some or all of the Client Hints header
fields. For example, avoid transmission of header fields that can
carry higher risks of linkability.
User agents MUST clear persisted opt-in preferences when any one of
site data, browsing cache, cookies, or similar are cleared.
Deployment and Security Risks
Deployment of new request headers requires several considerations:
- Potential conflicts due to existing use of a header field name
- Properties of the data communicated in a header field value
Authors of new Client Hints are advised to carefully consider
whether they need to be able to be added by client-side content (e.g.,
scripts) or whether the Client Hints need to be exclusively set by the user
agent. In the latter case, the Sec- prefix on the header field name
has the effect of preventing scripts and other application content
from setting them in user agents. Using the "Sec-" prefix signals to
servers that the user agent -- and not application content -- generated
the values. See for more
information.
By convention, request headers that are Client Hints are encouraged
to use a CH- prefix, to make them easier to identify as using this
framework; for example, CH-Foo or, with a "Sec-" prefix,
Sec-CH-Foo. Doing so makes them easier to identify programmatically
(e.g., for stripping unrecognized hints from requests by privacy
filters).
A Client Hints request header negotiated using the Accept-CH opt-in
mechanism MUST have a field name that matches sf-token
().
Abuse Detection
A user agent that tracks access to active fingerprinting
information SHOULD consider emission of Client Hints headers similar
to the way it would consider access to the equivalent API.
Research into abuse of Client Hints might look at how HTTP
responses to requests that contain Client Hints differ from those with
different values and from those without values. This might be used to reveal
which Client Hints are in use, allowing researchers to further analyze
that use.
Cost of Sending Hints
Sending Client Hints to the server incurs an increase in request byte
size. Some of this increase can be mitigated by HTTP header
compression schemes, but each new hint sent will still lead to some
increased bandwidth usage. Servers SHOULD take that into account when
opting in to receive Client Hints and SHOULD NOT opt-in to receive
hints unless they are to be used for content adaptation purposes.
Due to request byte size increase, features relying on this document
to define Client Hints MAY consider restricting sending those hints to
certain request destinations ,
where they are more likely to be useful.
IANA Considerations
Features relying on this document are expected to register added
request header fields in the "Permanent Message Header Field Names" registry
.
This document defines the "Accept-CH" HTTP response header field;
IANA has registered it in the same registry.
Accept-CH
- Header field name:
- Accept-CH
- Applicable protocol:
- HTTP
- Status:
- experimental
- Author/Change controller:
- IETF
- Specification document(s):
- of this
RFC
- Related information:
- for Client Hints
References
Normative References
Structured Field Values for HTTP
Informative References
User-Agent Client Hints
Google
Google
Client Hints Infrastructure
Google
Fetch - Living Standard
WHATWG
Acknowledgements
Thanks to , , ,
, , , , ,
, , and numerous other members of the IETF
HTTP Working Group for invaluable help and feedback.