Permissions Policy Guide (Previously Feature Policy)

Permissions policy is the new name for feature policy with a new HTTP header which uses structured header syntax.

How to add a new feature to permissions policy

Permissions policy (see spec) is a mechanism that allows developers to selectively enable and disable various browser features an APIs (e.g, “vibrate”, “fullscreen”, “usb”, etc.). A permissions policy can be defined via a HTTP header and/or an iframe “allow” attribute.

Below is an example of a header policy (note that the header should be kept in one line, split into multiple for clarity reasons):

Permissions-Policy: vibrate=(), geolocation=(self https://example.com), camera=*

• vibrate is disabled for all browsing contexts;
• geolocation is disabled for all browsing contexts except for its own origin and those whose origin is “https://example.com”;
• camera is enabled for all browsing contexts.

Below is an example of a container policy:

<iframe allowpaymentrequest allow='vibrate; fullscreen'></iframe>

OR

<iframe allowpaymentrequest allow="vibrate 'src'; fullscreen 'src'"></iframe>

• payment is enabled (via allowpaymentrequest) on all browsing contexts within the iframe;
• vibrate and fullscreen are enabled on the origin of the URL of the iframe's src attribute.

Combined with a header policy and a container policy, inherited policy defines the availability of a feature. See more details for how to define an inherited policy for feature

Adding a new feature to permissions policy

A step-to-step guide with examples.

Shipping features behind a flag

If the additional feature is unshipped, or if the correct behaviour with feature policy is undetermined, consider shipping the feature behind a runtime-enabled feature.

Define new feature

1. Permissions policy features are defined in third_party/blink/renderer/core/permissions_policy/permissions_policy_features.json5. Add the new feature, placing any runtime-enabled feature or origin trial dependencies in its “depends_on” field as described in the file's comments. This list is used to generate permissions_policy_helper.cc.

2. Append the new feature enum with a brief description as well in third_party/blink/public/mojom/permissions_policy/permissions_policy_feature.mojom. The enum must have the same name as the name field in the json5 file from step 1. Run tools/metrics/histograms/update_permissions_policy_enum.py to update enums.xml from the mojo enum.

3. Append the new feature name to the PermissionsPolicyFeature enum in third_party/blink/public/devtools_protocol/browser_protocol.pdl.

4. Send a Pull Request to the webappsec-permissions-policy github repo in order to propose the new permissions policy name. See: https://github.com/w3c/webappsec-permissions-policy/blob/main/features.md

Integrate the feature behaviour with permissions policy

1. The most common way to check if features are enabled is ExecutionContext::IsFeatureEnabled.

2. Examples:

• vibrate: NavigatorVibration::vibrate()
• payment: AllowedToUsePaymentRequest()
• usb: USB::getDevices()

Write web-platform-tests

To test the new feature with permissions policy, refer to third_party/blink/web_tests/external/wpt/permissions-policy/README.md for instructions on how to use the permissions policy test framework.

Document Policy Guide

How to add a new feature to document policy

Document Policy (see spec) is a similar mechanism to Feature Policy. It is intended

to cover those kinds of features which don't involve delegation of permission to trusted origins; features which are more about configuring a document, or removing features (sandboxing) from a document or a frame. Document Policy can only be set through HTTP header, and will not inherit to subframes.

Example HTTP header: Document-Policy: force-load-at-top=?0, lossy-images-max-bpp=1.0

• force-load-at-top is set to boolean value false (?0 in structured header syntax), i.e. the feature is disallowed in current document;
• lossy-images-max-bpp is set to 1.0, i.e. lossy image format (e.g. jpeg) images with byte per pixel rate higher than 1.0 will be blocked.

Adding a new feature to document policy

Shipping features behind a flag

If the additional feature is unshipped, or if the correct behaviour with document policy is undetermined, consider shipping the feature behind a runtime-enabled feature.

Define new feature

1. Document policy features are defined in third_party/blink/renderer/core/permissions_policy/document_policy_features.json5. Add the new feature, placing any runtime-enabled feature or origin trial dependencies in its “depends_on” field as described in the file's comments.

2. Append the new feature enum with a brief description as well in third_party/blink/public/mojom/permissions_policy/document_policy_feature.mojom

Integrate the feature behaviour with document policy

The most common way to check if features are enabled is ExecutionContext::IsFeatureEnabled.

Write web-platform-tests

Please add new tests to third_party/blink/web_tests/external/wpt/document-policy/.

Contacts

For more questions, please feel free to reach out to: iclelland@chromium.org (Emerita: loonybear@)