1. Introduction
The web-platform provides an ever-expanding set of features and APIs, offering richer functionality, better developer ergonomics, and improved performance. However, a missing piece is the ability for the developer to selectively enable, disable, or modify the behavior of some of these browser features and APIs within their application:
- The developer may want to selectively disable access to certain browser features and APIs to "lock down" their application, as a security or performance precaution, to prevent own and third-party content executing within their application from introducing unwanted or unexpected behaviors within their application.
- The developer may want to selectively enable access to certain browser features and APIs which may be disabled by default - e.g. some features may be disabled by default in embedded context unless explicitly enabled; some features may be subject to other policy requirements.
- The developer may want to use the policy to assert a promise to a client or an embedder about the use—or lack of thereof—of certain features and APIs. For example, to enable certain types of "fast path" optimizations in the browser, or to assert a promise about conformance with some requirements set by other embedders - e.g. various social networks, search engines, and so on.
This specification defines a feature policy mechanism that addresses the above use cases.
2. Examples
SecureCorp Inc. wants to disable use of Fullscreen and Geolocation APIs within their application. It can do so by delivering the following HTTP response header to define a feature policy:
Feature-Policy: fullscreen 'none'; geolocation 'none'
By specifying the "'none'
"keyword for the origin list, the
specified features will be disabled for all browsing contexts, regardless of
their origin.
SecureCorp Inc. wants to disable use of Geolocation API within all
browsing contexts except for its own origin and those whose origin is
"https://example.com
". It can do so by delivering the
following HTTP response header to define a feature policy:
Feature-Policy: geolocation 'self' https://example.com
The allowlist is a list of one or more origins, which can include
the application’s origin, optionally with the keyword "'self'
",
and any third-party origin.
SecureCorp Inc. is hosting an application on
"https://example.com
" and wants to disable camera and
microphone input on its own origin but enable it for a specific embedee
("https://other.com
"). It can do so by delivering the
following HTTP response header to define a feature policy:
Feature-Policy: camera https://other.com; microphone https://other.com
Some features are disabled by default in embedded contexts. The policy allows the application to selectively enable such features for specified origins.
FastCorp Inc. wants to disable geolocation for all cross-origin child frames, except for a specific iframe. It can do so by delivering the following HTTP response header to define a feature policy:
Feature-Policy: geolocation 'self'
and including an "allow
" attribute on the iframe
element:
<iframe src="https://other.com/map" allow="geolocation"></iframe>
Iframe attributes can selectively enable features in certain frames, and not in others, even if those contain documents from the same origin.
3. Other and related mechanisms
[HTML5] defines a sandbox
attribute for iframe
elements
that allows developers to reduce the risk of including potentially untrusted
content by imposing restrictions on content’s abilities - e.g. prevent it
from submitting forms, running scripts and plugins, and more. The sandbox directive defined by [CSP2] extends this capability to any
resource, framed or not, to ask for the same set of restrictions - e.g. via an
HTTP response header (Content-Security-Policy: sandbox
). These
mechanisms enable the developer to:
- Set and customize a sandbox policy on any resource via CSP.
- Set and customize individual sandbox policies on each
iframe
element within their application.
However, there are several limitations to the above mechanism: the developer cannot automatically apply a policy across all contexts, which makes it hard or impossible to enforce consistently in some cases (e.g. due to third-party content injecting frames, which the developer does not control); there is no mechanism to selectively enable features that may be off by default; the sandbox mechanism automatically disables all sandbox features, and requires the developer to opt back in to each of them, so it is impossible to extend the set of sandbox features without significant compatibility risk.
Feature Policy is intended to be used in combination with the sandbox mechanism (i.e. it does not duplicate feature controls already covered by sandbox), and provides an extensible mechanism that addresses the above limitations.
4. Framework
4.1. Policy-controlled Features
A policy-controlled feature is an API or behaviour which can be enabled or disabled in a document by referring to it in a feature policy.
Policy-controlled features are identified by tokens, which are character strings used in policy directives.
Each policy-controlled feature has a default allowlist, which defines whether that feature is available in documents in top-level browsing contexts, and how access to that feature is inherited in nested browsing contexts.
A user agent has a set of supported features, which is the set of features which it allows to be controlled through policies. User agents are not required to support every feature.
4.2. Policies
A feature policy is a struct with the following items:
- An inherited policy.
- A declared policy.
An empty feature policy is a feature policy that
has an inherited policy which contains "Enabled
" for
every supported feature, and a declared policy which is an
empty map.
4.3. Inherited policies
An inherited
policy is an ordered map from features to either
"Enabled
" or "Disabled
".
The inherited policy for a feature feature is the value in the inherited policy whose key is feature. After a feature policy has been initialized, its inherited policy will contain a value for each supported feature.
Each document in a frame tree inherits a set of policies from its parent
frame, or in the case of the top-level document, from the defined defaults
for each policy-controlled feature. This inherited policy determines
the initial state ("Enabled
" or "Disabled
") of
each feature, and whether it can be controlled by a declared policy in the document.
In a Document
in a top-level browsing context, the inherited
policy is based on defined defaults for each feature.
In a Document
in a nested browsing context, the inherited policy
is based on the parent document’s feature policy, as well as the nested
browsing context's container policy.
4.4. Declared policies
A declared policy is an ordered map from features to allowlists.
4.5. Header policies
A header policy is a list of policy directives delivered via an HTTP header with a document. This forms the document’s feature policy’s declared policy.
4.6. Container policies
In addition to the header policy, each nested browsing context has a container policy, which is a policy directive, which may be empty. The container policy can set by attributes on the browsing context container.
The container policy for a nested browsing context influences the inherited policy of any document loaded into that context. (See §9.8 Define an inherited policy for feature in document)
iframe
"allowfullscreen
",
"allowpaymentrequest
",
"and allow
" attributes.
Future revisions to this spec may introduce a mechanism to explicitly
declare the full container policy. 4.7. Policy directives
A policy directive is an ordered map, mapping policy-controlled features to corresponding allowlists of origins.
A policy directive is represented in HTTP headers and HTML attributes as its ASCII serialization.
4.8. Allowlists
A feature policy allowlist is conceptually a set of origins. An allowlist may be either:
- The special value
*
, which represents every origin, or - An ordered set of origins
'self'
, 'src'
, and 'none'
can appear in the text representation of allowlists in
headers and attribute strings. These keywords are always interpreted in
context during parsing, and only the origins which they refer to are
stored in the allowlist. The keywords themselves are not part of the
allowlist. To determine whether an allowlist matches an origin origin, run these steps:
- If the allowlist is the special value
*
, then return true. -
Otherwise, for each item in the allowlist:
- If item is same origin-domain with origin, then return true.
- return false.
4.9. Default Allowlists
Every policy-controlled feature has a default allowlist. The default allowlist determines whether the feature is allowed in a document with no declared policy in a top-level browsing context, and also whether access to the feature is automatically delegated to documents in nested browsing contexts.
The default allowlist for a feature is one of these values:
*
- The feature is allowed in documents in top-level browsing contexts by default, and when allowed, is allowed by default to documents in nested browsing contexts.
'self'
- The feature is allowed in documents in top-level browsing contexts by default, and when allowed, is allowed by default to same-origin domain documents in nested browsing contexts, but is disallowed by default in cross-origin documents in nested browsing contexts.
- 'none'
- The feature is disallowed in documents in top-level browsing contexts by default, and is also disallowed by default to documents in nested browsing contexts.
5. Feature Policy Serialization
5.1. ASCII serialization
Policy Directives are represented in HTTP headers and in HTML attributes as ASCII text.
serialized-feature-policy = serialized-policy-directive *(";" serialized-policy-directive) serialized-policy-directive = feature-identifier RWS allow-list feature-identifier = 1*( ALPHA / DIGIT / "-") allow-list = allow-list-value *(RWS allow-list-value) allow-list-value = serialized-origin / "*" / "'self'" / "'src'" / "'none'"
serialized-origin
is the serialization of an origin. However, the code points U+0027 ('),
U+0021 (*), U+002C (,) and U+003B (;) MUST NOT appear in the serialization.
If they are required, they must be percent-encoded as "%27
", "%2A
",
"%2C
" or "%3B
", respectively.
'self'
" may be used as an origin in an allowlist.
When it is used in this way, it will refer to the origin of the document
which contains the feature policy. 6. Delivery
6.1. Feature-Policy HTTP Header Field
The `Feature-Policy
` HTTP
header field can be used in the response (server to client) to
communicate the feature policy that should be enforced by the
client.
The header’s value is the §5.1 ASCII serialization of one or more policy directives:.
FeaturePolicy = serialized-feature-policy *("," serialized-feature-policy)
6.2. The allow
attribute of the iframe
element
iframe
elements have an "allow
" attribute, which
contains an ASCII-serialized policy
directive.
The allowlist for the features named in the attribute may be empty; in
that case, the default value for the allowlist is 'src'
, which
represents the origin of the URL in the iframe’s src
attribute.
When not empty, the "allow
" attribute will result in adding
an allowlist for each recognized feature to the nested browsing
context's container policy, when it is constructed.
6.3. Additional attributes to support legacy features
Some features controlled by Feature Policy have existing iframe attributes defined. This specification redefines these attributes to act as declared policies for the iframe element.
6.3.1. allowfullscreen
The "allowfullscreen
" iframe attribute controls access to requestFullscreen()
.
If the iframe element has an "allow
" attribute whose
value contains the token "fullscreen
", then the
"allowfullscreen
" attribute must have no effect.
Otherwise, the presence of an "allowfullscreen
" attribute
on an iframe will result in adding an allowlist of *
for the "fullscreen
" feature to the nested browsing
context's container policy, when it is constructed.
<iframe
allow="fullscreen">
, and is for compatibility with existing
uses of allowfullscreen
. If allow="fullscreen"
and allowfullscreen
are
both present on an iframe element, then the more restrictive allowlist
of allow="fullscreen"
will be used. 6.3.2. allowpaymentrequest
The "allowpaymentrequest
" iframe attribute controls
access to PaymentRequest.
If the iframe element has an "allow
" attribute whose
value contains the token "payment
", then the
"allowpaymentrequest
" attribute must have no effect.
Otherwise, the presence of an "allowpaymentrequest
"
attribute on an iframe will result in adding an allowlist of *
for the "payment
" feature to the nested
browsing context's container policy, when it is constructed.
<iframe
allow="payment">
, and is for compatibility with existing uses
of allowpaymentrequest
. If allow="payment"
and allowpaymentrequest
are both present on an iframe
element, then the more restrictive allowlist of allow="payment"
will be used. 7. Policy Introspection from Scripts
7.1. Overview
The current policy which is in effect in a document can be observed by scripts. This can be used to make decisions, for instance, about what user interface to display, in cases where it is not possible to determine otherwise whether a feature is enabled or not. (Some features may not have any observable failure mode, or may have unwanted side effects to feature detection.)
Documents and iframes both provide a FeaturePolicy
object which can be
used to inspect the feature policies which apply to them.
7.1.1. Document policies
To retreive the currently effective policy, use document.featurePolicy
. This returns a FeaturePolicy
object, which can be used to:
-
query the state (allowed or denied) in the current document for a given feature,
-
get a list of all available features (allowed or not) in the current document,
-
get a list of all allowed features in the current document, or
-
get the allowlist for a given feature in the current document.
<!doctype html> <script> const policy = document.featurePolicy; // This will be true if this document can use WebUSB. const can_use_usb = policy.allowsFeature('usb'); // True if a new frame at https://example.com will be allowed to use WebVR. if (policy.allowsFeature('vr', 'https://example.com')) { // Show UI to create frame at https://example.com. } else { // Show an alternative UI. } // Get the list of origins which are allowed to request payment. The result // will be a list of explicit origins, or the single element ['*'] if all // origins are allowed. const allowed_payment_origins = policy.getAllowlistForFeature('payment'); // Get the list of all features supported in this document (even those // which are not allowed). The result will be an array of strings, each // representing a feature. const all_features = policy.features(); if (all_features.includes('geolocation')) { // Append a child frame to a third-party map service. } </script>
7.1.2. Frame policies
It is also possible to inspect the policy on an iframe element, from the document which contains it. The policy object in this case represents the observable policy for the frame, which depends only on the current document and the attributes of the iframe element. It does not reveal whether a feature is actually currently allowed in the frame, as the document in the frame may have applied its own policy via an HTTP header, or may have navigated away from its initial location to a new origin. Revealing the effective policy in the nested browsing context in that case could leak information about the behaviour of a cross-origin document.
<!doctype html> <iframe id="frame" allow="fullscreen; vr"></iframe> <script> const iframe_element = document.getElementById("frame"); const iframe_policy = iframe_element.featurePolicy; // True if the framed document will be allowed to use WebVR if (iframe_policy.allowsFeature('vr')) { // display vr controls } </script>
The observable policy on an iframe element is independent of any actual content loaded into the frame (to avoid cross-origin information leakage,) or even whether it is in a document tree.
<!doctype html> <!-- this frame should not be allowed to use fullscreen when the document in its src attribute is loaded in it --> <iframe id="frame" allow="fullscreen https://example.com" src="https://example.net/" ></iframe> <script> const iframe_element = document.getElementById("frame"); const iframe_policy = iframe_element.featurePolicy; // This will be false, as the URL listed in the src attribute is not allowed // by policy to use fullscreen. const is_fullscreen_allowed_in_frame = iframe_policy.allowsFeature('fullscreen'); const new_frame = document.createElement('iframe'); new_frame.allow = 'syncxhr'; // This will be true, as the iframe is allowed to use syncxhr at whatever URL is // mentioned in its src attribute, even though that attribute is not yet set. const is_syncxhr_allowed = new_frame.featurePolicy.allowsFeature('syncxhr'); </script>
7.2. The featurePolicy object
[NoInterfaceObject ]interface {
FeaturePolicy boolean (
allowsFeature DOMString ,
feature optional DOMString );
origin sequence <DOMString >();
features sequence <DOMString >();
allowedFeatures sequence <DOMString >(
getAllowlistForFeature DOMString ); };
feature partial interface Document { [SameObject ]readonly attribute FeaturePolicy ; };
featurePolicy partial interface HTMLIFrameElement { [SameObject ]readonly attribute FeaturePolicy ; };
featurePolicy
A FeaturePolicy
object has an associated node, which is a Node
. The associated node is set when the FeaturePolicy
object is created.
A FeaturePolicy
object has a default origin, which is an origin, whose value depends on the state of the FeaturePolicy
object’s associated node:
-
If the
FeaturePolicy
object’s associated node is aDocument
, then its default origin is theDocument
's
-
If the
FeaturePolicy
object’s associated node is anElement
, then its default origin is theElement
's
Each Document
has a policy object, which is
a FeaturePolicy
instance whose associated node is that Document
.
A Document
's featurePolicy
IDL attribute, on getting,
must return the Document
's policy object.
Each iframe
element has a policy object,
which is a FeaturePolicy
instance whose associated node is that
element.
An iframe
's featurePolicy
IDL attribute, on
getting, must return the iframe
's policy object.
The allowsFeature(feature, origin)
method must run the following
steps:
-
If origin is omitted, set origin to this
FeaturePolicy
object’s default origin. -
Let policy be the observable policy for this
FeaturePolicy
object’s associated node. -
If feature is allowed by policy for origin, return true.
-
Otherwise, return false.
The features()
method must run the following steps:
-
Set result to an empty ordered set.
-
For each supported feature feature:
-
Append feature to result.
-
-
return result
The allowedFeatures()
method must run the following steps:
-
Set result to an empty ordered set.
-
Let origin be this
FeaturePolicy
object’s default origin. -
Let policy be the observable policy for this
FeaturePolicy
object’s associated node. -
For each supported feature feature:
-
If feature is allowed by policy for origin, append feature to result.
-
-
return result
The getAllowlistForFeature(feature)
method must run the following
steps:
-
Set result to an empty list
-
Let origin be this
FeaturePolicy
object’s default origin. -
Let policy be the observable policy for this
FeaturePolicy
object’s associated node. -
If feature is not allowed in policy for origin, return result
-
Let allowlist be policy’s declared policy[feature]
-
If allowlist is the special value
*
, append "*
" to result -
Otherwise, for each origin in allowlist:
-
Append the serialization of origin to result
-
-
Return result.
The observable policy for any Node is a feature policy, which contains the information about the policy in the browsing context represented by that Node which is visible from the current browsing context.
To get the observable policy for a Document document, return document’s feature policy.
To get the observable policy for an Element node, run the following steps:
-
Let inherited policy be a new ordered map.
-
Let declared policy be a new ordered map.
-
For each supported feature feature:
-
Let isInherited be the result of running §9.9 Define an inherited policy for feature in container at origin on feature, node and node’s declared origin.
-
Set inherited policy[feature] to isInherited.
-
-
Return a new feature policy with inherited policy inherited policy and declared policy declared policy.
To get the declared origin for an Element node, run the following steps:
-
If node’s node document’s sandboxed origin browsing context flag is set, then return a unique opaque origin.
-
If node’s
sandbox
attribute is set, and does not contain theallow-same-origin
keyword, then return a unique opaque origin. -
If node’s
srcdoc
attribute is set, then return node’s node document’s origin. -
If node’s
src
attribute is set:-
Let url be the result of parsing node’s src attribute, relative to node’s node document.
-
If url is not failure, return url’s origin.
-
-
Return node’s node document’s origin.
The declared origin concept is intended to represent the origin of
the document which the embedding page intends to load into a frame. This
means, for instance, that if the browser does not support the sandbox
or srcdoc
attributes, it should not take
those attributes into account when computing the declared origin.
8. Reporting
Feature policy violation reports indicate that some behavior of the Document has violated a feature policy. It is up to each individual feature policy to define/determine when a violation of that policy has occurred.
Feature policy violation reports have the report type "feature-policy-violation".
Feature policy violation reports are visible to ReportingObserver
s.
interface :
FeaturePolicyViolationReportBody ReportBody {readonly attribute DOMString ;
featureId readonly attribute DOMString ?;
sourceFile readonly attribute long ?;
lineNumber readonly attribute long ?;
columnNumber readonly attribute DOMString ; };
disposition
A feature policy violation report’s body, represented in
JavaScript by FeaturePolicyViolationReportBody
, contains the following
fields:
-
featureId: The string identifying the policy-controlled feature whose policy has been violated. This string can be used for grouping and counting related reports.
-
sourceFile: If known, the file where the violation occured, or null otherwise.
-
lineNumber: If known, the line number in sourceFile where the violation occured, or null otherwise.
-
columnNumber: If known, the column number in sourceFile where the violation occured, or null otherwise.
-
disposition: A string indicating whether the violated feature policy was enforced in this case. disposition will be set to "enforce" if the policy was enforced, or "report" if the violation resulted only in this report being generated (with no further action taken by the user agent in response to the violation).
Note: There is currently no mechanism in place for enabling report-only mode, so disposition will always be set to "enforce".
9. Algorithms
9.1. Process response policy
Given a response (response) and an origin (origin), this algorithm returns a declared feature policy.
- Abort these steps if the response’s header list does not contain a header whose name is "
Feature-Policy
". - Let header be the concatenation of the values of all header fields in response’s header list whose name is
"
Feature-Policy
", separated by U+002C (,) (according to [RFC7230, 3.2.2]). - Let feature policy be the result of executing §9.2 Parse header from value and origin on header and origin.
- Return feature policy.
9.2. Parse header from value and origin
Given a string (value) and an origin (origin) this algorithm will return a declared feature policy.
- Let policy be an empty ordered map.
-
For each element returned by splitting value on commas:
- Let directive be the result of executing §9.3 Parse policy directive on element with container origin set to origin.
- Run §9.4 Merge directive with declared policy on directive and policy.
- Return policy.
9.3. Parse policy directive
Given a string (value), an origin (container origin), and an optional origin (target origin), this algorithm returns a policy directive.
- Let directive be an empty ordered map.
-
For each serialized-declaration returned by strictly splitting value on the delimiter
U+003B (;):
- Let tokens be the result of splitting serialized-declaration on ASCII whitespace.
- If tokens is an empty list, then continue.
- Let feature-name be the first element of tokens.
- If feature-name does not identify any recognized policy-controlled feature, then continue.
- Let feature be the policy-controlled feature identified by feature-name.
- Let targetlist be the remaining elements, if any, of tokens.
- Let allowlist be a new allowlist.
- If any element of targetlist is the string
"
*
", set allowlist to the special value*
. -
Otherwise:
- Set allowlist to an new ordered set.
- If targetlist is empty and target origin is given, append target origin to allowlist.
-
For each element in targetlist:
- If element is an ASCII case-insensitive match for "
'self'
", let result be container origin. - If target origin is given, and element is an ASCII case-insensitive match
for "
'src'
", let result be target origin. - Otherwise, let result be the result of executing the URL parser on element.
-
If result is not failure:
- Let target be the origin of result.
- If target is not an opaque origin, append target to allowlist.
- If element is an ASCII case-insensitive match for "
- Set directive[feature] to allowlist.
- Return directive
9.4. Merge directive with declared policy
Given a policy directive (directive) and a declared policy (policy), this algorithm will modify policy to account for the new directive.
-
For each feature → allowlist of directive:
- If policy does not contain an allowlist for feature, then set policy[feature] to allowlist.
9.5. Process feature policy attributes
Given an element (element), this algorithm returns a container policy, which may be empty.
- Let policy be a new policy directive.
- Let container policy be the result of running Parse policy directive on the value of element’s
allow
attribute, with container origin set to the origin of element’s node document, and target origin set to element’s declared origin. -
If element is an
iframe
element:-
If element’s
allowfullscreen
attribute is specified, and container policy does not contain an allowlist forfullscreen
,- Construct a new declaration for
fullscreen
, whose allowlist is the special value*
. - Add declaration to container policy.
- Construct a new declaration for
-
If element’s
allowpaymentrequest
attribute is specified, and container policy does not contain an allowlist forpayment
,- Construct a new declaration for
payment
, whose allowlist is the special value*
. - Add declaration to container policy.
- Construct a new declaration for
-
If element’s
- Return container policy.
9.6. Initialize document’s Feature Policy
Given a Document
object (document), this algorithm
initialises document’s Feature Policy
- Let inherited policy be a new ordered map.
- Let declared policy be a new ordered map.
-
For each feature supported,
- Let isInherited be the result of running §9.8 Define an inherited policy for feature in document on feature and document.
- Set inherited policy[feature] to isInherited.
- Let policy be a new feature policy, with inherited policy inherited policy and declared policy declared policy.
- Enforce the policy policy on document.
9.7. Initialize document’s Feature Policy from response
Given a response (response) and a Document (document), this algorithm populates document’s Feature Policy
- Initialize document’s Feature Policy
- Let inherited policy be document’s Feature Policy’s inherited policy.
- Let declared policy be a new ordered map.
- Let d be the result of running Process response policy on response and document’s origin.
-
For each feature → allowlist of d:
- If inherited policy[feature] is true, then set declared policy[feature] to allowlist.
- Let policy be a new feature policy, with inherited policy inherited policy and declared policy declared policy.
- Enforce the policy policy on document.
9.8. Define an inherited policy for feature in document
Given a feature (feature) and a Document (document), this algorithm returns the inherited policy for that feature.
- Let context be document’s browsing context.
- If context is a nested browsing context, return the result of executing §9.9 Define an inherited policy for feature in container at origin for feature in context’s browsing context container at document’s origin.
- Otherwise, return "
Enabled
".
9.9. Define an inherited policy for feature in container at origin
Given a feature (feature) a browsing context container (container), and an origin for a document in that container (origin), this algorithm returns the inherited policy for that feature.
- Let parent be container’s node document.
- Let container policy be the result of running §9.5 Process feature policy attributes on container.
-
If feature is a key in container policy:
- If the allowlist for feature in container policy does not match origin,
return "
Disabled
". - If feature is enabled in parent for parent’s origin,
return "
Enabled
".
- If the allowlist for feature in container policy does not match origin,
return "
- If feature is
enabled in parent for origin, return
"
Enabled
". - Otherwise return "
Disabled
".
9.10. Is feature enabled in document for origin?
Given a feature (feature), a Document
object
(document), and an origin (origin), this algorithm
returns "Disabled
" if feature should be considered
disabled, and "Enabled
" otherwise.
- Let policy be document’s Feature Policy
- If policy’s inherited policy for feature is Disabled, return "
Disabled
". -
If feature is present in policy’s declared
policy:
- If the allowlist for feature in policy’s declared policy matches origin, then return "
Enabled
". - Otherwise return "
Disabled
".
- If the allowlist for feature in policy’s declared policy matches origin, then return "
- If feature’s default allowlist is
*
, return "Enabled
". - If feature’s default allowlist is
'self'
, and origin is same origin-domain with document’s origin, return "Enabled
". - Return "
Disabled
".
9.11. Generate report for violation of feature policy on settings
Given a feature (feature), an environment settings object (settings), and an optional string (group), this algorithm generates a report about the violation of the policy for feature.
-
Let body be a new
FeaturePolicyViolationReportBody
, initialized as follows:- featureId
-
feature’s string representation.
- sourceFile
-
null
- lineNumber
-
null
- columnNumber
-
null
- disposition
-
"enforce"
-
If the user agent is currently executing script, and can extract the source file’s URL, line number, and column number from settings, then set body’s sourceFile, lineNumber, and columnNumber accordingly.
-
If group is omitted, set group to "default".
-
Execute Reporting API §#queue-report with body, "feature-policy-violation", group, and settings.
Note: This algorithm should be called when a feature policy has been violated.
9.12. Should request be allowed to use feature?
Given a feature (feature) and a request (request), this algorithm returns true
if the request should be allowed to use feature, and false
otherwise.
- Set window to request’s window.
-
If window is not a
Window
, returnfalse
.Feature Policy withinno-window
contexts (WorkerGlobalScope
orWorkletGlobalScope
) is being figured out in issue #207. After that’s resolved, update this algorithm to allow fetches initiated within these contexts to use policy-controlled features. Until that’s resolved, disallow all policy-controlled features (e.g., sending Client Hints to third parties) in these contexts. - Set document to window’s associated
Document
. - Let origin be request’s origin.
- Let result be the result of executing Is feature enabled in document for origin? on feature, document, and origin.
- If result is "
Enabled
", returntrue
. - Otherwise, return
false
.
10. IANA Considerations
The permanent message header field registry should be updated with the following registration [RFC3864]:
- Header field name
- Feature-Policy
- Applicable protocol
- http
- Status
- standard
- Author/Change controller
- W3C
- Specification document
- Feature Policy API
11. Privacy and Security
This specification standardizes a mechanism for an embedding page to set a
policy which will be enforced on an embedded page. Similar to iframe sandbox
, this can be done without the express permission of the
embedded page, which means that behaviors of existing features can be changed
in published web sites, by embedding them in another document with an
appropriate container policy.
As such, the biggest privacy and security concerns are:
- Exposure of behavior in a cross-origin subframe to its embedder
- Unanticipated behavior changes in subframes controlled by the embedder
To a degree, these concerns are already present in the web platform, and this specification attempts to at least not make them needlessly worse.
Security and privacy issues may also be caused by the design of individual features, so care must be taken when integrating with this specification. This section attempts to provide some guidance as to what kinds of behaviors could cause such issues.
11.1. Exposure of cross-origin behavior
Features should be designed such that a violation of the policy in a framed document is not observable by documents in other frames. For instance, a hypothetical feature which caused a event to be fired in the embedding document if it is used while disabled by policy, could be used to extract information about the state of an embedded document. If the feature is known only to be used while a user is logged in to the site, for instance, then the embedder could disable that feature for the frame, and then listen for the resulting events to determine whether or not the user is logged in.
The introspection API is designed to only show information about a
subframe’s policy which could already be deduced by the embedding document.
This observable policy is not affected by any HTTP headers delivered
with the framed document, and does not change when the frame navigates itself,
even if such navigation is to a different origin, where a different policy
applies. Only navigations caused by setting the src
attribute of the <iframe>
element will cause the observable policy to be updated.
11.2. Unanticipated behavior changes
Feature policy grants a document the ability to control which features will and will not be availble in a subframe at the time it is loaded. When a feature represents an existing, long-standing behavior of the web platform, this may mean that existing published content on the web was not written with the expectation that a particular API could fail.
As a practical (though contrived) example, consider a document which uses synchronous XMLHttpRequest to determine whether a user has sufficient privileges to access the page:
<!DOCTYPE html> <h1>Welcome to SecureCorp!</h1> <script> var req = new XMLHttpRequest(); req.open("GET", "/api/security_check.json", false); req.send(); if (req.response == "untrusted user") { // User is not logged in; redirect to a safe page location.href = "/security_check_failed.html"; } </script> <!-- Page continues with assumption that user is logged in -->
If this document is embedded by a page which disables the
"sync-xhr
" feature, the call to XMLHttpRequest.open()
would
fail, and the security check would be bypassed.
Note that this sort of behavior forcing is already possible on the web: some features are only allowed in top-level documents, and not in any iframes, and iframe sandboxing can be used in a similar way to embed a frame without access to features which it may be depending on.
In general, this concern is mitigated in two ways:
- The vulnerable page may be served with an
X-Frame-Options
HTTP header which does not allow it to be framed by an attacker. - Sites should use feature detection to determine whether an API or behavior is available before attempting to use it, and should handle any documented errors returned or exceptions thrown by the APIs they call.
- In the case where feature detection is not possible, new web content can
be written to use the
policy
object to inspect the feature policy which is currently enforced, and adjust behaviour or user interface accordingly.
Authors integrating their features with Feature Policy can decide when and how the feature will fail when a document attempts to use it while it is disabled. Authors should attempt to make use of existing failure modes, when they exist, to increase the chance that existing content will already be correctly handling such failures.
11.3. Exposure of embedding policy
Care has been taken to limit the information which an page can infer about the behavior of cross-origin pages which it embeds. It may be possible in some scenarios however, for the embedded page to infer information about its embedder, by examining the policy which the embedder has enforced on it.
This is similar to the existing document.fullscreenEnabled
property,
which can be used by the embedded document to infer whether its embedder has
granted it the ability to use the Fullscreen API. If this is only granted in
certain cases — when the user is logged in to the embedding site, for
instance — then the embedded site can learn something about the state of
its embedder.