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 policy mechanism that addresses the above use cases.
This specification used to be named Feature Policy.
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 permissions policy:
Permissions-Policy: fullscreen=(), geolocation=()
By specifying an empty origin list, the specified features will be disabled for all documents, including nested documents, regardless of their origin.
Geolocation is disabled by default in all cross-origin frames. FastCorp
Inc. has a specific cross-origin iframe on their site for which it wants to
enable geolocation. It can do so by 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.
SecureCorp Inc. wants to completely disable use of the Geolocation API
within all descendant navigables except for its own origin and
those whose origin is "https://example.com
", even in the
presence of an attacker who can embed their own iframes on SecureCorp’s
pages. It can do this by delivering the following HTTP response header to
define a restricted permissions policy for Geolocation:
Permissions-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.
With this policy in effect, it can then use the "allow
"
iframe attribute as usual to grant geolocation to certain frames, but only
those frames hosting content from http://example.com or SecureCorp itself
will actually be granted the ability to use that API.
SecureCorp Inc. restructured its domains and now needs to needs to delegate
use of the Geolocation API to its origin ("https://example.com
")
as well as three subdomains ("https://geo.example.com
",
"https://geo2.example.com
", and "https://new.geo2.example.com
").
This needs to be accomplished while still disabling the use of the Geolocation API
within all other browsing contexts. It can do this by delivering the following HTTP response header:
Permissions-Policy: geolocation=(self "https://example.com" "https://geo.example.com" "https://geo2.example.com" "https://new.geo2.example.com")
This works, but if SecureCorp Inc. feels safe delegating to any subdomains on
"https://example.com
" the HTTP response header could instead be:
Permissions-Policy: geolocation=(self "https://example.com" "https://*.example.com")
Not only would the above header permit "https://geo.example.com
",
"https://geo2.example.com
", and "https://new.geo2.example.com
"
to use the Geolocation API, but any other subdomains of "https://example.com
"
could use it too. Note that "https://example.com
" is not covered by the allowlist entry "https://*.example.com
" and must also be added.
SecureCorp Inc. restructured its services and now needs to needs to delegate
use of the Geolocation API to its origin ("https://example.com
")
as well as three non-default ports ("https://example.com:444
",
"https://example.com:445
", and "https://example.com:446
").
This needs to be accomplished while still disabling the use of the Geolocation API
within all other browsing contexts. It can do this by delivering the following HTTP response header:
Permissions-Policy: geolocation=(self "https://example.com" "https://example.com:444" "https://example.com:445" "https://example.com:446")
This works, but if SecureCorp Inc. feels safe delegating to any ports on
"https://example.com
" the HTTP response header could instead be:
Permissions-Policy: geolocation=(self "https://example.com:*")
Not only would the above header permit "https://example.com:444
",
"https://example.com:444
", and "https://example.com:445
"
to use the Geolocation API, but any other ports on "https://example.com
"
could use it too.
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.
Permissions 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 permissions 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 traversables, and how access to that feature is inherited in child navigables.
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 permissions policy is a struct with the following items:
- inherited policy
-
an ordered map from features to "
Enabled
" or "Disabled
" - declared policy
-
an ordered map from features to allowlists
An empty permissions policy is a permissions
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
The inherited policy for a feature feature is the value in the inherited policy whose key is feature. After a permissions policy has been initialized, its inherited policy will contain a value for each supported feature.
Upon both creation and navigation, Each Document
inherits a set of
policies from its parent frame, or in the case of the Document
in a top-level traversable, 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 traversable, the inherited
policy is based on defined defaults for each feature.
In a Document
in a child navigable, the inherited policy is based
on the parent document’s permissions policy, as well as the child
navigable's container policy.
4.4. Header policies
A header policy is a list of policy directives delivered via an HTTP header with a document. This forms the document’s permissions policy’s declared policy.
4.5. Container policies
In addition to the header policy, each child navigable has a container policy, which is a policy directive, which may be empty. The container policy can set by attributes on the navigable container.
The container policy for a child navigable influences the inherited policy of any Document
loaded into
that navigable. (See § 9.7 Define an inherited policy for feature in container at origin).
iframe
allowfullscreen
, and allow
attributes. Future revisions to this spec may introduce a
mechanism to explicitly declare the full container policy. 4.6. 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 as the serialization of an sh-dictionary structure, and in and HTML attributes as its ASCII serialization.
4.7. Allowlists
A permissions policy allowlist is conceptually a set of origins. An allowlist may be either:
- The special value
*
, which represents every origin, or - A struct containing:
- expressions, which is an ordered set of permissions-source-expression
- self-origin, which is an origin or
null
- src-origin, which is an origin or
null
'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.
Note: We are not using the CSP variant of wildcard matching as it requires the HTTPS scheme.
-
If the allowlist’s self-origin is not null and it is same origin-domain with origin, then return true.
-
If the allowlist’s src-origin is not null and it is same origin-domain with origin, then return true.
-
If origin is an opaque origin, return false.
-
Let url be the result of calling the url parser on the serialization of origin.
-
For each permissions-source-expression item in the allowlist’s expressions:
-
If the result of running Does url match expression in origin with redirect count? on url, item, origin, and 0 is true then return true.
-
-
Return false.
4.8. 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
traversable, and also whether access to the feature is automatically
delegated to documents in child navigables.
The default allowlist for a feature is one of these values:
*
- The feature is allowed in
Document
s in top-level traversables by default, as well as those in all child navigables. It can be disallowed in child navigables by explicitly supplying a container policy on the navigable container that overrides this default (or in any navigable, by delivering theDocument
with a suitablePermissions-Policy
header). 'self'
- The feature is allowed in documents in top-level traversables by default, as well as those in child
navigables whose document is same origin with its parent's document, when allowed in that
Document
. It is disallowed by default in child navigables whose document is cross-origin with its parent's document.
5. Permissions Policy Serialization
5.1. HTML attribute serialization
Policy Directives in HTML attributes are represented as their ASCII serialization, with the following ABNF:
serialized-permissions-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 = permissions-source-expression / "*" / "'self'" / "'src'" / "'none'" permissions-source-expression = scheme-source / host-source
'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 permissions
policy. 5.2. Structured header serialization
Policy Directives in HTTP headers are represented as Structured Fields. [HEADER-STRUCTURE]In this representation, a policy directive is represented by a Dictionary.
Each Dictionary Member associates a feature with an allowlist. The Member Names must be Tokens. If a token does not name a supported feature, then the Dictionary Member will be ignored by the processing steps.
The Member Values represent allowlists, and must be one of:
-
a String containing the ASCII permissions-source-expression
-
the Token
*
-
the Token
self
-
an Inner List containing zero or more of the above items.
Any other items inside of an Inner List will be ignored by the processing steps, and the Member Value will be processed as if they were not present. Member Values of any other form will cause the entire Dictionary Member to be ignored by the processing steps.
6. Delivery
6.1. Permissions-Policy HTTP Header Field
The `Permissions-Policy
`
HTTP header field can be used in the response (server to client) to
communicate the permissions policy that should be enforced by the
client.
Permissions-Policy is a structured header. Its value must be a dictionary. It’s ABNF is:
PermissionsPolicy = sh-dictionary
The semantics of the dictionary are defined in § 5.2 Structured header serialization.
The processing steps are defined in § 9.2 Construct policy from dictionary and origin.
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 iframe
element’s content navigable's container policy, when it is
constructed.
6.3. Additional attributes to support legacy features
Some features controlled by
Permissions Policy have existing iframe attributes defined. This
specification redefines these attributes to influence the iframe
's content navigable's container policy.
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 iframe
element’s content navigable'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. 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 PermissionsPolicy
object which can
be used to inspect the permissions policies which apply to them.
7.1.1. Document policies
To retreive the currently effective policy, use document.permissionsPolicy
. This returns a PermissionsPolicy
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.permissionsPolicy; // 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 WebXR. if (policy.allowsFeature('xr-spatial-tracking', '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 iframe element’s nested navigable in that case could leak information about the behaviour of a cross-origin document.
<!doctype html> <iframe id="frame" allow="fullscreen; xr-spatial-tracking"></iframe> <script> const iframe_element = document.getElementById("frame"); const iframe_policy = iframe_element.permissionsPolicy; // True if the framed document will be allowed to use WebXR if (iframe_policy.allowsFeature('xr-spatial-tracking')) { // display virtual reality 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.permissionsPolicy; // 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 = 'sync-xhr'; // This will be true, as the iframe is allowed to use sync-xhr at whatever URL is // mentioned in its src attribute, even though that attribute is not yet set. const is_sync_xhr_allowed = new_frame.permissionsPolicy.allowsFeature('sync-xhr'); </script>
7.2. The permissionsPolicy object
[Exposed =Window ]interface {
PermissionsPolicy boolean (
allowsFeature DOMString ,
feature optional DOMString );
origin sequence <DOMString >();
features sequence <DOMString >();
allowedFeatures sequence <DOMString >(
getAllowlistForFeature DOMString ); };
feature partial interface Document { [SameObject ]readonly attribute PermissionsPolicy ; };
permissionsPolicy partial interface HTMLIFrameElement { [SameObject ]readonly attribute PermissionsPolicy ; };
permissionsPolicy
A PermissionsPolicy
object has an associated node, which
is a Node
. The associated node is set when the PermissionsPolicy
object is created.
A PermissionsPolicy
object has a default origin, which is
an origin, whose value depends on the state of the PermissionsPolicy
object’s associated node:
-
If the
PermissionsPolicy
object’s associated node is aDocument
, then its default origin is theDocument
's origin. -
If the
PermissionsPolicy
object’s associated node is anElement
, then its default origin is theElement
's declared origin.
Each Document
has a policy object, which is
a PermissionsPolicy
instance whose associated node is that Document
.
A Document
's permissionsPolicy
IDL attribute, on
getting, must return the Document
's policy object.
Each iframe
element has a policy object,
which is a PermissionsPolicy
instance whose associated node is
that element.
An iframe
's permissionsPolicy
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
PermissionsPolicy
object’s default origin. -
Let policy be the observable policy for this
PermissionsPolicy
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
PermissionsPolicy
object’s default origin. -
Let policy be the observable policy for this
PermissionsPolicy
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
PermissionsPolicy
object’s default origin. -
Let policy be the observable policy for this
PermissionsPolicy
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 -
Return result.
-
-
If the allowlist’s self-origin is not null, append the serialization of it to result
-
If the allowlist’s src-origin is not null, append the serialization of it to result
-
Otherwise, for each permissions-source-expression item in allowlist’s expressions:
-
Append item to result
-
-
Return result.
The observable policy for any Node is a permissions policy, which contains the information about the policy in the navigable represented by that Node which is visible from the current document.
To get the observable policy for a Document document, return document’s permissions policy.
To get the observable policy for an Element node, run the following steps:
-
Let inherited policy be an empty ordered map.
-
For each supported feature feature:
-
Let isInherited be the result of running 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 permissions policy with inherited policy inherited policy and declared policy a new ordered map.
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 new opaque origin.
-
If node’s
sandbox
attribute is set, and does not contain theallow-same-origin
keyword, then return a new 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
Permissions policy violation reports indicate that some behavior of the Document has violated a permissions policy. It is up to the specification of each individual policy-controlled feature to define what it means to violate that policy, and how to determine when such a violation has occurred.
Permissions policy violation reports have the report type "permissions-policy-violation".
Permissions policy violation reports are visible to ReportingObserver
s.
[Exposed =Window ]interface :
PermissionsPolicyViolationReportBody ReportBody {readonly attribute DOMString ;
featureId readonly attribute DOMString ?;
sourceFile readonly attribute long ?;
lineNumber readonly attribute long ?;
columnNumber readonly attribute DOMString ; };
disposition
A permissions policy violation report’s body, represented in
JavaScript by PermissionsPolicyViolationReportBody
, 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 permissions 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
-
Let parsed header be the result of executing get a structured field value given "
Permissions-Policy
" and "dictionary" from response’s header list. -
If parsed header is null, return an empty ordered map.
-
Let policy be the result of executing Construct policy from dictionary and origin on parsed header and origin.
-
Return policy.
9.2. Construct policy from dictionary and origin
-
Let policy be an empty ordered map.
-
For each feature-name → value of dictionary:
-
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 allowlist be a new allowlist.
-
If value is the token
*
, or if value is a list which contains the token*
, set allowlist to the special value*
. -
Otherwise:
-
If value is the token
self
, let allowlist’s self-origin be origin. -
Otherwise if value is a list, then for each element in value:
-
If element is the token
self
, let allowlist’s self-origin be origin. -
If element is a valid permissions-source-expression, append element to allowlist’s expressions.
-
-
-
Set policy[feature] to allowlist.
-
-
Return policy.
9.3. Parse 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:
-
If targetlist is empty and target origin is given, let allowlist’s src-origin be target origin.
-
For each element in targetlist:
-
If element is an ASCII case-insensitive match for "
'self'
":-
Let allowlist’s self-origin be container origin.
-
Continue to the next element.
-
-
If target origin is given, and element is an ASCII case-insensitive match for "
'src'
":-
Let allowlist’s src-origin be target origin.
-
Continue to the next element.
-
-
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 the serialization of target to allowlist’s expressions.
-
-
-
-
Set directive[feature] to allowlist.
-
-
Return directive
9.4. Process permissions policy attributes
-
If element is not an
iframe
element, then return an empty policy directive. -
Let container policy be the result of running Parse policy directive given the value of element’s
allow
attribute, the origin of element’s node document, and element’s declared origin. -
If element’s
allowfullscreen
attribute is specified, and container policy does not contain an entry for thefullscreen
feature.-
Set container policy[
fullscreen
] = the special value*
.
-
-
Return container policy.
9.5. Create a Permissions Policy for a navigable
-
Assert: If not null, container is a navigable container.
-
Let inherited policy be a new ordered map.
-
-
Let isInherited be the result of running Define an inherited policy for feature in container at origin on feature, container and origin.
-
Set inherited policy[feature] to isInherited.
-
-
Let policy be a new permissions policy, with inherited policy inherited policy and declared policy a new ordered map.
-
Return policy.
9.6. Create a Permissions Policy for a navigable from response
-
Let policy be the result of running Create a Permissions Policy for a navigable given container and origin.
-
Let d be the result of running Process response policy on response and origin.
-
For each feature → allowlist of d:
-
If policy’s inherited policy[feature] is true, then set policy’s declared policy[feature] to allowlist.
-
-
Return policy.
9.7. Define an inherited policy for feature in container at origin
Document
in
that container (origin), this algorithm returns the inherited policy value for feature.
-
If container is null, return "
Enabled
". -
If the result of executing Get feature value for origin on feature, container’s node document, and container’s node document’s origin is "
Disabled
", return "Disabled
". -
If the result of executing Get feature value for origin on feature, container’s node document, and origin is "
Disabled
", return "Disabled
". -
Let container policy be the result of running Process permissions policy attributes on container.
-
If feature exists in container policy:
-
If feature’s default allowlist is
*
, return "Enabled
". -
If feature’s default allowlist is
'self'
, and origin is same origin with container’s node document’s origin, return "Enabled
". -
Otherwise return "
Disabled
".
9.8. Get feature value for origin
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 permissions 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
".
-
-
Return "
Enabled
".
9.9. Is feature enabled in document for origin?
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 permissions 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 feature’s default allowlist is
*
, return "Enabled
". -
If feature’s default allowlist is
'self'
, and origin is same origin with document’s origin, return "Enabled
". -
Return "
Disabled
".
9.10. Generate report for violation of permissions policy on settings
-
Let body be a new
PermissionsPolicyViolationReportBody
, 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 generate and queue a report with body, "permissions-policy-violation", group, and settings.
Note: This algorithm should be called when a permissions policy has been violated.
9.11. Should request be allowed to use feature?
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
.Permissions Policy within non-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
- Permissions-Policy
- Applicable protocol
- http
- Status
- standard
- Author/Change controller
- W3C
- Specification document
- Permissions 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
The Permissions Policy mechanism 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 permissions policy which is currently enforced, and adjust behaviour or user interface accordingly.
Authors integrating their features with Permissions 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.
12. Change log
12.1. Changes since FPWD
-
Expose new algorithms to create a Feature Policy before document is created. Link
-
Remove algorithms no longer needed. Link
-
Change same-origin-domain check to same-origin. Link
-
Change Header and attribute combination from OR to AND semantics. Link
-
Rename to "Permissions Policy". Link
-
Define "Permissions-Policy" as a structured header. Link
-
Editorial fixes.
-
The
allowpaymentrequest
attribute was removed due to deprecation by Payment Request API and HTML. Link