Storage Access API

Browsers implement several storage access features and policies that restrict access to unpartitioned cookies by embedded, cross-site resources. These range from giving embedded resources under each top-level origin a unique cookie storage space (partitioned cookies), to outright blocking of cookie access when resources are loaded in a third-party context.

The semantics around unpartitioned cookie blocking features/policies differ from browser to browser, but the core functionality is similar: cross-site resources embedded in a third-party context are not given access to the same cookies that they would have access to when loaded in a first-party context. This is done with good intent — browser vendors want to take steps to better protect their user's privacy and security, for example leaving them less open to having their activity tracked across different sites, and less vulnerable to exploits such as cross-site request forgery (CSRF).

However, there are legitimate uses for embedded cross-site content accessing unpartitioned cookies, which the above features/policies are known to break. As an example, federated logins often require access to authentication cookies, and will require the user to sign in on each site separately (or completely break) if those cookies are not available. As a result, site owners often encourage users to add their site as an exception or to disable such policies entirely. Users who wish to continue to interact with embedded content are forced to greatly relax their blocking policy for resources loaded from all embedded origins and possibly across all websites.

The Storage Access API is intended to solve this problem; embedded cross-site content can request unrestricted access to unpartitioned cookies on a frame-by-frame basis, for a particular top-level embedding site, via the Document.requestStorageAccess() method. It can also check whether it already has access via the Document.hasStorageAccess() method.

As explained above, modern browsers implement various features and policies to restrict or block access to unpartitioned cookies by embedded content. Such content that has a legitimate need for cookie access can get around this problem using the Storage Access API as follows:

1. It can call the Document.hasStorageAccess() method to check whether it has the access it needs already.
2. If not, it can request access via the Document.requestStorageAccess() method.
3. Depending on the browser, the user will be asked whether to grant access to the requesting embed in slightly different ways.
   • Safari shows prompts for all embedded content that has not previously received storage access.
   • Firefox only prompts users after an origin has requested storage access on more than a threshold number of sites.
   • At the time of writing, Chrome doesn't prompt the user. Instead, it only resolves requestStorageAccess() calls that come from domains within a first-party set. This behavior will change as the implementation is developed further.
4. Access is granted or denied based on whether the content meets all the security requirements (see Security measures, below). The Promise-based nature of requestStorageAccess() allows you to run code to handle success and failure cases.
   • Modern spec behavior dictates that access is granted per-frame — every separate content embed has its unpartitioned cookie access blocked by default, and needs to call requestStorageAccess() to opt-in to access. If a content embed has received access, and same-site embeds then call requestStorageAccess(), their promises will fulfill automatically. But they still need to opt-in.
   • In older versions of the spec, the access was per-page. As soon as one embed received unpartitioned cookie access via requestStorageAccess(), all other same-site embeds would automatically receive access. This was not desirable behavior from a security standpoint — for example, if shop.example.com embedded locator.users.com to allow users to use their location info while shopping, and locator.users.com called requestStorageAccess(), shop.example.com and any other sites it embeds would be able to access its cookies, but also access cookies from private.users.com, which is not intended to be embedded. Read more about the motivations behind this change.
   • Consult the Browser compatibility information to see which browsers implement which behavior.
5. Once access is granted, a permission key is stored in the browser with the structure <top-level site, embedded site>. For example, if the embedding site is embedder.com, and the embed is locator.example.com, the key would be <embedder.com, example.com>. Same-site embeds (docs.example.com, profile.example.com, etc.) would then be able to call requestStorageAccess() and the promise would fulfill automatically, as mentioned earlier.
   • Older versions of the spec used the more specific permission key structure <top-level site, embedded origin>, which meant that same-site, cross-origin embeds didn't match the permission key and had to go through the whole process separately.
   • At the time of writing, only Chromium browsers had implemented the new behavior.

Several different security measures could cause a Document.requestStorageAccess() call to fail. Check the below list if you are having trouble getting a request to work:

1. The call must be associated with a user gesture (transient activation) such as a tap or click. This prevents embedded content on the page from spamming the browser or user with excessive access requests. Note that this isn't required if:
   • Permission to use the API has already been granted, for example by another same-site resource calling requestStorageAccess().
   • The caller is a top-level document or same-site to the top-level document. In such cases, requestStorageAccess() probably doesn't need to be called at all.
2. The document and top-level document must not have a null origin.
3. Origins that have never been interacted with as a first party do not have a notion of first-party storage. From the user's perspective, they only have a third-party relationship with that origin. Access requests are automatically denied if the browser detects that the user hasn't interacted with the embedded content in a first-party context recently (in Firefox, "recently" means within 30 days).
4. The document's window must be a secure context.
5. Sandboxed <iframe>s cannot be granted storage access by default for security reasons. The API therefore also adds the allow-storage-access-by-user-activation sandbox token. The embedding website needs to add this to allow storage access requests to be successful, along with allow-scripts and allow-same-origin to allow it to execute a script to call the API and execute it in an origin that can have cookies:

   html

   <iframe
     sandbox="allow-storage-access-by-user-activation
                   allow-scripts
                   allow-same-origin">
     …
   </iframe>
   

6. Usage of this feature may be blocked by a storage-access Permissions Policy set on your server.
7. At the time of writing, Chrome only resolves requestStorageAccess() calls that come from domains within a first-party set. This restriction is intended to be relaxed as the implementation is developed further.

Although the API surface is the same, websites using the Storage Access API should expect differences in the level and extent of unpartitioned cookie access they receive between different browsers, due to differences in their storage access policies.

Design properties unique to Firefox are summarized here:

• If the embedded origin tracker.example has already obtained unpartitioned cookie access on the top-level origin foo.example, and the user visits a page from foo.example embedding a page from tracker.example again in less than 30 days, the embedded origin will have unpartitioned cookie access immediately when loading.
• If an embedded page from tracker.example has previously successfully obtained unpartitioned cookie access on top-level origin foo.example, all embedded subresources from tracker.example on foo.example (e.g. scripts, images, stylesheets, etc.) will load with access to their cookies, which means they may send Cookie headers and honor incoming Set-Cookie headers.
• In Firefox, when the promise returned from requestStorageAccess() is resolved, the embedded page will gain access to its entire first-party storage, not just unpartitioned cookies. This includes access to APIs such as Web Storage, IndexedDB, DOM Cache, and so on.
• The storage access grants are phased out after 30 calendar days have passed.

Documentation for Firefox's new storage access policy for blocking tracking cookies includes a detailed description of the scope of storage access grants.

• The storage access grants are phased out after 30 days of browser usage passed without user interaction. Successful use of the Storage Access API resets this counter.

See Using the Storage Access API for an implementation guide with code examples.

Document.hasStorageAccess()

Returns a Promise that resolves with a boolean value indicating whether the document has access to unpartitioned cookies.

Document.requestStorageAccess()

Returns a Promise that resolves if the access to unpartitioned cookies was granted, and rejects if access was denied.

Permissions.query()

You can call Permissions.query() with a value of "storage-access" to query whether unpartitioned cookie access has been granted in general, i.e. to another same-site embed. If so, you can call requestStorageAccess() without user interaction, and the promise will resolve automatically.

• Using the Storage Access API
• Introducing Storage Access API (WebKit blog)