Storage Access API

Chrome is phasing out support for third-party cookies to reduce cross-site tracking. This poses a challenge for sites and services that rely on cookies in embedded contexts, for user journeys such as authentication. The Storage Access API (SAA) allows these use cases to continue to work, while limiting cross-site tracking as much as possible.

Implementation status

Browser Support

  • 119
  • 85
  • 65
  • 11.1

Source

The Storage Access API is available in all major browsers, but there are slight implementation differences between browsers. These differences have been highlighted in the relevant sections in this post.

Work is continuing to resolve all remaining blocking issues, before standardizing the API.

What is the Storage Access API?

The Storage Access API is a JavaScript API that allows iframes to request storage access permissions when access would otherwise be denied by browser settings. Embeds with use cases that depend on loading cross-site resources can use the API to request access permission from the user, on an as-needed basis.

If the storage request is granted, then the iframe will have access to its cross-site cookies, which are also available when users visit it as a top-level site.

The Storage Access API allows specific cross-site cookie access to be provided with minimal burden to the end user, while still preventing generic cross-site cookie access as is often used for user tracking.

Use cases

Some third-party embeds require access to cross-site cookies to provide a better experience to the user—something that won't be available after third-party cookies have been deprecated.

Use cases include:

  • Embedded commenting widgets which require login session details.
  • Social media "Like" buttons which require login session details.
  • Embedded documents which require login session details.
  • A premium experience provided to a video embed (for example, to not show ads for logged in users, or to know the user's preferences for closed captions or restrict certain video types).
  • Embedded payment systems.

Many of these use cases involve persisting login access in embedded iframes.

When to use the Storage Access API over other APIs

The Storage Access API is one of the alternatives to using third-party cookies, so it is important to understand when to use this API compared to the others. It's meant for use cases where both the following are true:

  • The user will interact with the embedded content—that is, it is not a passive iframe or a hidden iframe.
  • The user has visited the embedded origin in a top-level context—that is, when that origin is not embedded in another site.

There are alternative APIs for a variety of use cases:

  • Cookies Having Independent Partitioned State (CHIPS) allows developers to opt-in a cookie to "partitioned" storage, with a separate cookie jar per top-level site. For example, a third-party web-chat widget may rely on setting a cookie to save session information. The session information is saved per-site, so the cookie set by the widget doesn't need to be accessed on other websites where it is also embedded. The Storage Access API is useful when an embedded third-party widget is dependent on sharing the same information across different origins (for example for logged-in session details or preferences).
  • Related Websites Sets (RWS) is a way for an organization to declare relationships among sites, so that browsers allow limited third-party cookie access for specific purposes. Sites still need to request access with the Storage Access API, but for sites within the set, access can be granted without user prompts.
  • Federated Credential Management (FedCM) is a privacy-preserving approach to federated identity services. The Storage Access API deals with accessing cookies post-login. For some use cases FedCM provides an alternative solution to the Storage Access API, and may be preferable as it features a more login-oriented browser prompt. However, adopting FedCM usually requires additional changes to your code, for example to support its HTTP endpoints.
  • Anti-fraud, ad-related, and measurement APIs also exist, and the Storage Access API is not intended to address those concerns.

Using the Storage Access API

The Storage Access API has two promised-based methods:

It also integrates with the Permissions API. This lets you check the status of the storage-access permission in a third-party context, which indicates whether a call to document.requestStorageAccess() would be automatically granted:

Using the hasStorageAccess() method

When a site first loads, it can use the hasStorageAccess() method to check whether access to third-party cookies has already been granted.

// Set a hasAccess boolean variable which defaults to false.
let hasAccess = false;

async function handleCookieAccessInit() {
  if (!document.hasStorageAccess) {
    // Storage Access API is not supported so best we can do is
    // hope it's an older browser that doesn't block 3P cookies.
    hasAccess = true;
  } else {
    // Check whether access has been granted via the Storage Access API.
    // Note on page load this will always be false initially so we could be
    // skipped in this example, but including for completeness for when this
    // is not so obvious.
    hasAccess = await document.hasStorageAccess();
    if (!hasAccess) {
      // Handle the lack of access (covered later)
    }
  }
  if (hasAccess) {
    // Use the cookies.
  }
}
handleCookieAccessInit();

Storage access is only granted to an iframe document after it calls requestStorageAccess(), so hasStorageAccess() will always return false initially—except when another same-origin document in the same iframe had already been granted access. The grant is preserved across same-origin navigations inside the iframe specifically to allow reloads after granting access for pages that require cookies to be present in the initial request for the HTML document.

Using the requestStorageAccess() method

If the iframe does not have access it may need to request access using the requestStorageAccess() method:

if (!hasAccess) {
  try {
    await document.requestStorageAccess();
  } catch (err) {
    // Access was not granted and it may be gated behind an interaction
    return;
  }
}

The first time this is requested, the user may need to approve this access with a browser prompt, after which the promise will resolve, or will reject resulting in an exception if await is used.

To prevent abuse, this browser prompt will only be shown after a user interaction. That's why requestStorageAccess() initially needs to be called from a user-activated event handler, rather than immediately as the iframe loads:

async function doClick() {

  // Only do this extra check if access hasn't already been given
  // based on the hasAccess variable.
  if (!hasAccess) {
    try {
      await document.requestStorageAccess();
      hasAccess = true; // Can assume this was true if above did not reject.
    } catch (err) {
      // Access was not granted.
      return;
    }
  }

  if (hasAccess) {
    // Use the cookies
  }
}

document.querySelector('#my-button').addEventListener('click', doClick);

Permission prompts

When the user clicks the button for the first time, the browser prompt will automatically appear, typically in the address bar. Below is an example of Chrome's prompt, but other browsers have a similar UI:

Screenshot of the Chrome Storage Access API permission prompt
Chrome's Storage Access API permission prompt

The prompt may be skipped by the browser and permission automatically provided in certain circumstances:

  • If the page and iframe have been used in the last 30 days after accepting the prompt.
  • If the embedded iframe is part of a Related Website Set.
  • In Firefox, the prompt is also skipped for known websites (those you have interacted with at the top level) for the first five attempts.

Alternatively, the method may be automatically rejected without showing the prompt in certain circumstances:

  • If the user has not previously visited and interacted with the site that owns the iframe as a top-level document, not in an iframe. This means the Storage Access API is only useful for embedded sites that users have previously visited in a first-party context.
  • If the requestStorageAccess() method is called outside of a user interaction event without prior approval of the prompt after an interaction.

While the user will be prompted on the initial use, subsequent visits can resolve requestStorageAccess() without a prompt and without requiring user interaction in Chrome and Firefox. Note that Safari always requires a user interaction.

As cookie access may be granted without a prompt, or user interaction, it is often possible to get third-party cookie access before a user interaction on browsers that support this (Chrome and Firefox) by calling requestStorageAccess() on page load. This may allow you to access third-party cross-site cookies immediately and provide a fuller experience, even before the user interacts with the iframe. This can be a better user experience for some situations than waiting for user interaction.

Using the storage-access permission query

To check whether access can be granted without a user interaction, you can check the status of the storage-access permission and only make the requestStoreAccess() call early if no user action is required, rather than call it and have it fail when an interaction is required.

This also lets you potentially handle the need for a prompt upfront by displaying different content—for example, a login button.

The following code adds the storage-access permission check to the earlier example:

// Set a hasAccess boolean variable which defaults to false except for
// browsers which don't support the API - where we assume
// such browsers also don't block third-party cookies.
let hasAccess = false;

async function hasCookieAccess() {
  // Check if Storage Access API is supported
  if (!document.requestStorageAccess) {
    // Storage Access API is not supported so best we can do is
    // hope it's an older browser that doesn't block 3P cookies.
    return true;
  }

  // Check if access has already been granted
  if (await document.hasStorageAccess()) {
    return true;
  }

  // Check the storage-access permission
  // Wrap this in a try/catch for browsers that support the
  // Storage Access API but not this permission check
  // (e.g. Safari and older versions of Firefox).
  let permission;
  try {
    permission = await navigator.permissions.query(
      {name: 'storage-access'}
    );
  } catch (error) {
    // storage-access permission not supported. Assume no cookie access.
    return false;
  }

    if (permission) {
    if (permission.state === 'granted') {
      // Permission has previously been granted so can just call
      // requestStorageAccess() without a user interaction and
      // it will resolve automatically.
      try {
        await document.requestStorageAccess();
        return true;
      } catch (error) {
        // This shouldn't really fail if access is granted, but return false
        // if it does.
        return false;
      }
    } else if (permission.state === 'prompt') {
      // Need to call requestStorageAccess() after a user interaction
      // (potentially with a prompt). Can't do anything further here,
      // so handle this in the click handler.
      return false;
          } else if (permission.state === 'denied') {
            // Currently not used. See:
      // https://github.com/privacycg/storage-access/issues/149
      return false;
          }
    }

  // By default return false, though should really be caught by one of above.
  return false;
}

async function handleCookieAccessInit() {
  hasAccess = await hasCookieAccess();

  if (hasAccess) {
    // Use the cookies.
  }
}

handleCookieAccessInit();

Sandboxed iframes

When using the Storage Access API in sandboxed iframes, the following sandbox permissions are required:

  • allow-storage-access-by-user-activation is required to allow access to the Storage Access API.
  • allow-scripts is required to allow use of JavaScript to call the API.
  • allow-same-origin is required to allow access to same-origin cookies and other storage.

For example:

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

To be accessed with the Storage Access API in Chrome, cross-site cookies must be set with the following two attributes:

  • SameSite=None - which is required to mark the cookie as cross-site
  • Secure - which ensures only cookies set by HTTPS sites can be accessed.

In Firefox and Safari, cookies are defaulted to SameSite=None and they do not restrict SSA to Secure cookies so these attributes are not required. It is recommended to be explicit about the SameSite attribute and to always use Secure cookies.

Top-level page access

The Storage Access API is intended for enabling access to third-party cookies within embedded iframes.

There are also other use cases when the top-level page requires access to third-party cookies. For example, images or scripts which are restricted by cookies, which site owners may want to include directly in the top-level document rather than in an iframe. To address this use case Chrome has proposed an extension to the Storage Access API which adds arequestStorageAccessFor() method.

The requestStorageAccessFor() method

Browser Support

  • 119
  • 119
  • x
  • x

Source

requestStorageAccessFor() method works in a similar way to requestStorageAccess() but for top-level resources. It can only be used for sites within a Related Website Set to prevent granting general access to third-party cookies.

For more details on how to use requestStorageAccessFor() read the Related Website Sets: developer guide.

The top-level-storage-access permission query

Browser Support

  • 119
  • 119
  • x
  • x

Similar to the storage-access permission, there is a top-level-storage-access permission to check whether access can be granted for requestStorageAccessFor().

How is the Storage Access API different when used with RWS?

When Related Website Sets are used with the Storage Access API, certain additional capabilities are available as detailed in the following table:

Without RWS With RWS
Requires a user gesture to initiate the request for storage access
Requires user to visit requested storage origin in a top-level context before granting access
First time user prompt can be skipped
requestStorageAccess not required to be called if access has been previously granted
Automatically grants access across other domains in a Related Website Site
Supports requestStorageAccessFor for top-level page access
Differences between using Storage Access API without and with Related Website Sets

Demo: setting and accessing cookies

The following demo shows how a cookie set by yourself in the first screen of the demo can be accessed in an embedded frame in the second site of the demo:

storage-access-api-demo.glitch.me

The demo requires a browser with third-party cookies disabled:

  • Chrome 118 or higher with the chrome://flags/#test-third-party-cookie-phaseout flag set and browser restarted.
  • Firefox
  • Safari

Resources