Declare permissions and warn users

An extension's ability to access websites and most Chrome APIs is determined by its declared permissions. Permissions should be restricted to only what is needed for its functionality. Limiting permissions establishes an extension's capabilities and reduces possible incursion to data if the extension is compromised by an attacker. Protect extensions and their users by implementing explicit, minimal and optional permissions.

# Organize permissions

Permissions are known strings that refer to a Chrome API or match patterns that grant access to one or more hosts. They are listed in the manifest and specified as required permissions or optional permissions.

{
  "name": "Permissions Extension",
  ...
  "permissions": [
    "activeTab",
    "storage"
  ],
  "optional_permissions": [
    "topSites",
  ],
  "host_permissions": [
    "https://www.developer.chrome.com/*"
  ],
  "optional_host_permissions": [
    "http://*/*", "https://*/*"
  ],
  ...
  "manifest_version": 3
}

Limit required permissions to only what is needed for the extension's core functionality. An extension should not request more permissions than it needs; do not future-proof by requesting permissions that may be needed in a future update.

Permissions needed for optional features should be registered as optional permissions. This allows users to decide how much access they are willing to provide an extension and which features are desired.

# Identify required permissions

A simple extension may need to request multiple permissions, and many permissions display warnings on installation. Users are more likely to trust an extension with limited warnings or when permissions are explained to them.

Identify the core functionality of an extension and what permissions are required for it. Consider making features optional if they require permissions with warnings.

# Trigger optional permissions with events

The optional permissions sample extension's core functionality is overriding the new tab page. One feature is displaying the user's goal of the day. This feature only requires the storage permission, which does not include a warning.

The extension has an additional feature; displaying the user's top sites. This feature requires the topSites permission, which has a warning.

Developing features that rely on permissions with warnings as optional and introducing those features organically gives users a risk-free introduction to the extension. Additionally, this allows users to further customize their experience with an extension and creates an opportunity to explain warnings.

# Substitute the activeTab permission

The "activeTab" permission grants temporary access to the site the user is on and allows the extension to use host permissions on the current tab. It replaces "<all_urls>" in many cases and displays no warning on installation.

For details, see Using the activeTab permission.

# Allowing access

If an extension needs to access file:// URLs or operate in incognito mode, users will need to enable access for those features inside the extension's detail page at chrome://extensions.

An extension can detect if it is enabled in incognito mode by calling extension.isAllowedIncognitoAccess() or able run on file:// URLs with extension.isAllowedFileSchemeAccess().

# Understanding permissions

Permission warnings exist to describe the capabilities granted by an API to extension users, but some of these warnings may not be obvious at first. For instance, adding the "tabs" permission results in a seemingly unrelated warning: the extension can Read your browsing activity. Although the Tabs API might be used to only open new tabs, it can also be used to see the URL that is associated with every newly opened tab by using their tabs.Tab objects.

When possible, implement optional permissions or a less powerful API to avoid alarming warnings.

# Viewing warnings

No permission warnings will be displayed if an extension is loaded as an unpacked file. To view an extension's permission warnings, navigate to chrome://extensions, ensure developer mode is enabled and click PACK EXTENSION.

Specify the path to the extension's folder in the Extension root directory field then click the Pack Extension button. Ignore the Private key field for a first-time package.

Chrome will create two files, a .crx file and a .pem file. The .pem file contain the private key used to sign the extension.

Do not lose the private key! Keep the .pem file in a secret and secure place; it will be needed to update the extension.

Install the .crx file by dropping it into the Chrome Extension's Management page.

After dropping the .crx file the browser will ask if the extension can be added and display warnings.

# Permissions with warnings

Some permissions may not display warnings when paired with other permissions. For example, the "tabs" warning will not show if the extension also requests "<all_urls>".

The permissions warning table is updated on a best-effort basis and may contain slight discrepancies with the current warnings. To verify the most recent warnings shown for extension permissions, follow the steps in Viewing Warnings.

# Update permissions

When an extension is updated to include a new permission that triggers a warning it may temporarily disable it. The user will have to re-enable it after agreeing to any new warnings.

This can be avoided by making the new feature optional and adding new permission updates to optional_permissions in the manifest.