[whatwg/dom] Proposal: Storage Access API (#560)

Hi! John Wilander from WebKit here. We hope this can extend existing specifications rather than some whole new spec.

# Storage Access API

## Problem

**Tl;dr:** Browsers that block access to third-party cookies break authenticated embeds such as commenting widgets and subscribed video services.

In the context of cross-origin resource loads, cookies are popularly referred to as third-party cookies. In reality, these cookies are often the same as the first-party cookies so what we really mean with third-party cookies is access to first-party cookies in a third-party context.

A browser may have rules for third-party cookies that go beyond cookie policy rules such as scheme, host, path, secure attribute etc. These additional rules may be:

- Third-parties aren't allowed to set cookies,
- Third-parties have their cookies partitioned or double-keyed, or
- Third-parties have no cookie access.

However, certain services are intended to be embedded as third-party content and need access to first-party cookies for authentication. Examples are commenting widgets, video embeds, document embeds, and social media action widgets. These break if the third-party content has no access to its first-party cookies.

The same problem exists for other kinds of storage such as IndexedDB and LocalStorage, except they are not tied to the HTTP protocol and are typically not used for authentication purposes. From here on we will refer to cookies except when the distinction between cookies and other storage makes sense.

## Proposed Solution

**Tl;dr:** A new API with which cross-origin iframes can request access to their first-party cookies when processing a user gesture such as a tap or a click. This allows third-party embeds to authenticate on user interaction.

We propose two new functions on the document:

```
partial interface Document {
    Promise<bool> hasStorageAccess();
    Promise<void> requestStorageAccess();
};
```
hasStorageAccess() can be called at any time to check whether access is already granted and it doesn't require user interaction.

requestStorageAccess() should only be called on user interaction such as a tap or a click. It will check a set of rules and grant access if the rules are fulfilled. Access to first-party cookies in the given iframe can be assumed if the returned promise resolves. From that point, any sub resource load in the iframe will have first-party cookies sent and incoming cookies will be set in the first-party cookie jar.

Note that no other third-party resources on that webpage are affected by the storage access status of an individual iframe.

## Algorithm for requestStorageAccess()

1. If the document already has been granted access, resolve.
2. If the document has a null origin, reject.
3. If the document's frame is the main frame, resolve.
4. If the sub frame's origin is equal to the main frame's, resolve.
5. If the sub frame is not sandboxed, skip to step 7.
6.  If the sub frame doesn't have the token "allow-storage-access-by-user-activation", reject.
7. If the sub frame's parent frame is not the top frame, reject.
8. If the browser is not processing a user gesture, reject.
9. Check any additional rules that the browser has. Examples: Whitelists, blacklists, on-device classification, user settings, anti-clickjacking heuristics, or prompting the user for explicit permission. Reject if some rule is not fulfilled.
10. Grant the document access to cookies and store that fact for the purposes of future calls to hasStorageAccess() and requestStorageAccess().

## Access Removal

Storage access is granted for the life of the document and as long as the document's frame is attached to the DOM. This means:

* Access is removed when the sub frame navigates.
* Access is removed when the sub frame is detached from the DOM.
* Access is removed when the top frame navigates.
* Access is removed when the webpage goes away, such as a tab close.

In addition, the browser may decide to remove access on a timeout basis or on some dedicated user action such as switching cookie policy.

## WebKit Specifics

WebKit's implementation of Storage Access API will be available in Safari Technology Preview soon and on by default. It only covers cookie access, i.e. no other storage mechanisms and the partitioning of them is affected by a call to requestStorageAccess() at this point.


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