`document.hasStorageAccess()` False Negatives In A(B(A)) Embeds

Hey everyone! Let's talk about a peculiar issue with the document.hasStorageAccess() function, specifically its behavior in nested iframe scenarios like A(B(A)). If you're scratching your head, don't worry; we'll break it down. The core problem? It seems document.hasStorageAccess() might be incorrectly returning false in situations where it should be true, particularly when dealing with nested iframes from the same origin. This can have implications for how we manage storage access in web applications, so let's get into the nitty-gritty.

The Issue Unpacked: Why False Negatives?

The heart of the matter lies in the specification of document.hasStorageAccess() and how it interacts with the Storage Access API. The function is designed to tell us whether a document has access to its storage (like cookies) in a cross-site context. To understand why it's failing in certain cases, we need to dissect the conditions under which it's supposed to return true.

According to the specification (https://privacycg.github.io/storage-access/#dom-document-hasstorageaccess), there are primarily two scenarios where document.hasStorageAccess() should return true:

1. Same Authority Carveout (Step 10.3.5): This is a somewhat vaguely defined condition, but it essentially refers to situations where the embedded document and the top-level document are considered to be under the “same authority.” The specification's note suggests this relates to “same site checks while adhering to additional security aspects such as the presence of a cross-site parent document.” The crucial point here is whether nested documents, like the inner and outer “A” in an A(B(A)) frame tree, qualify as having the “same authority.”

   • In the A(B(A)) scenario, the interpretation is that the inner and outer "A" documents likely do not fall under the "same authority" carveout. This is because the presence of the intermediate “B” frame introduces a cross-site context, even though the outer and inner frames share the same origin. Therefore, the algorithm is unlikely to return true at this step for such a frame tree.

2. Granted Permission State (Step 10.3.6): This scenario depends on the permissionState being granted. This permissionState is retrieved from the permission store. Thus the only way for the state to be granted is if a script has previously requested permission to use the “storage-access” permission, and this request resulted in a permission store entry being set with the appropriate key (matching the top-level site and the embedded site).

   • The process of request permission to use is initiated by calling document.requestStorageAccess(). However, a critical check exists within document.requestStorageAccess() that impacts this scenario. Step 16.7 of the algorithm checks if the embedded site and the top-level site are same-site. If they are, the algorithm aborts early. This means that if the embedded site and the top-level site are both “A” (as in our A(B(A)) example), the request permission to use invocation becomes unreachable.
   • The Consequence: Because the permission request is never fully processed, the permission state remains at its default (likely prompt), preventing document.hasStorageAccess() from returning true. This is the crux of the issue: the intended mechanism for granting storage access in same-site nested iframe scenarios is effectively blocked by this early abort condition.

In essence, the current specification and implementation create a situation where, for A(B(A))-type embeddings, document.hasStorageAccess() is always destined to return false. This is because neither the “same authority” carveout nor the “granted permission state” mechanism can be triggered successfully in this specific scenario. This discrepancy is not just a theoretical concern; it has practical implications for how websites can leverage the Storage Access API in nested iframe structures.

Diving Deeper: The A(B(A)) Frame Tree and Storage Access

Let's break down the A(B(A)) scenario. Imagine you have a website "A.com". Now, within A.com, you embed an iframe from "B.com". Inside that B.com iframe, you embed another iframe, but this time it's back to A.com. This is the A(B(A)) structure.

The intention behind the Storage Access API is to allow the inner A.com to gain access to its cookies, even though it's embedded within B.com. This is crucial for scenarios where A.com might be providing a service or widget that needs to maintain user sessions or preferences across different websites. Without storage access, the embedded A.com would be essentially stateless, leading to a degraded user experience.

However, as we've seen, the current implementation of document.hasStorageAccess() throws a wrench in the works. Because of the same-site check within document.requestStorageAccess(), the inner A.com iframe can't successfully request storage access, even though it should be able to. This leads to document.hasStorageAccess() always returning false, even if the script has managed to gain storage access through other means (like a successful call to document.requestStorageAccess() before the same-site check was introduced).

This inconsistency is problematic. It means that developers can't rely on document.hasStorageAccess() to accurately reflect the storage access status of their embedded iframes. They might end up implementing workarounds or making incorrect assumptions about storage availability, leading to potential bugs and security vulnerabilities.

The Test Case: Proof in the Pudding

To further illustrate the issue, there's a web platform test (https://github.com/web-platform-tests/wpt/blob/b84f9bdaa5e087aa5fe434d9c1bbf008176951cf/storage-access-api/requestStorageAccess.sub.https.window.js#L47) that demonstrates that document.requestStorageAccess() can successfully resolve and grant access to cookies in an A(B(A)) frame tree. This test essentially proves that the underlying mechanism for granting storage access is working correctly. However, the fact that document.hasStorageAccess() still returns false in this scenario highlights the disconnect between the intended behavior and the actual implementation.

This test case serves as a concrete example of the false negative problem. It shows that document.hasStorageAccess() is not accurately reflecting the storage access state in A(B(A)) embeddings. This discrepancy needs to be addressed to ensure that the Storage Access API functions as expected and that developers have a reliable way to determine storage access status.

Why This Matters: Implications and Consequences

The erroneous behavior of document.hasStorageAccess() in A(B(A)) embeds isn't just a minor technicality; it has significant implications for web developers and the overall functionality of the Storage Access API.

• Inaccurate Storage Access Detection: The primary purpose of document.hasStorageAccess() is to provide a reliable way for embedded iframes to determine if they have access to their storage. When it returns false incorrectly, developers are left in the dark. They might assume that storage access is not available, even when it is, leading to suboptimal application behavior. Imagine an embedded widget that needs to display personalized content based on cookies. If document.hasStorageAccess() returns false, the widget might fall back to a generic, unpersonalized view, even though the user's data is actually accessible.

• Workarounds and Complexity: To compensate for the unreliable document.hasStorageAccess(), developers might resort to complex workarounds. They might try to infer storage access based on other signals, like the success or failure of specific API calls. These workarounds add unnecessary complexity to the code and can be fragile, potentially breaking if the underlying browser behavior changes. Furthermore, relying on workarounds can increase the risk of introducing bugs and security vulnerabilities.

• Confusion and Frustration: The inconsistency between the intended behavior of document.hasStorageAccess() and its actual behavior can lead to confusion and frustration among developers. They might spend significant time debugging issues related to storage access, only to discover that the root cause is a false negative from document.hasStorageAccess(). This wasted effort can hinder development productivity and slow down the adoption of the Storage Access API.

• Impact on User Experience: Ultimately, the incorrect behavior of document.hasStorageAccess() can negatively impact the user experience. If embedded iframes can't reliably access their storage, features like personalized content, seamless logins, and persistent preferences might not work correctly. This can lead to a degraded and inconsistent experience for users, especially when interacting with websites that rely heavily on embedded content.

In short, the false negative issue with document.hasStorageAccess() undermines the core purpose of the Storage Access API. It makes it harder for developers to build robust and user-friendly web applications that leverage embedded content. Addressing this issue is crucial for ensuring the long-term success and adoption of the API.

The Desired Behavior: A Path Forward

So, what's the ideal behavior for document.hasStorageAccess() in A(B(A)) embeds? The consensus seems to be that it should return true if the script has successfully gained storage access, even if the same-site check within document.requestStorageAccess() would normally prevent a new request from succeeding. This aligns with the overall intent of the Storage Access API, which is to allow legitimate cross-site interactions while protecting user privacy.

If a script in the inner A.com iframe has previously called document.requestStorageAccess() and the request has resolved successfully (granting access to cookies), then document.hasStorageAccess() should reflect this state. It shouldn't matter that a subsequent call to document.requestStorageAccess() might be blocked by the same-site check. The key is that storage access has already been granted, and document.hasStorageAccess() should accurately report this.

This desired behavior has several benefits:

• Accurate Reflection of Storage Access: It ensures that document.hasStorageAccess() provides an accurate snapshot of the current storage access state. This allows developers to make informed decisions about how to handle storage in their embedded iframes.
• Reduced Complexity: It eliminates the need for complex workarounds to detect storage access. Developers can rely on document.hasStorageAccess() as a single source of truth, simplifying their code and reducing the risk of bugs.
• Improved User Experience: By ensuring that embedded iframes can reliably access their storage, it enables a smoother and more consistent user experience. Features like personalized content and seamless logins can work as expected, even in nested iframe scenarios.

To achieve this desired behavior, the specification and implementation of document.hasStorageAccess() might need to be adjusted. One potential approach is to modify the algorithm to check if storage access has been granted at any point in the past, rather than just relying on the current permission state. This would ensure that document.hasStorageAccess() returns true even if a subsequent call to document.requestStorageAccess() would be blocked.

Conclusion: Towards a More Reliable Storage Access API

The issue of document.hasStorageAccess() returning false negatives in A(B(A)) embeds highlights a subtle but significant challenge in the Storage Access API. While the API aims to provide a balance between user privacy and legitimate cross-site interactions, this particular behavior creates an inconsistency that can hinder developers and degrade the user experience.

By understanding the root cause of the issue – the same-site check within document.requestStorageAccess() and its impact on the permission state – we can work towards a solution. The desired behavior is clear: document.hasStorageAccess() should accurately reflect whether storage access has been granted, regardless of whether a new request would be blocked.

Addressing this issue will require careful consideration of the specification and implementation of the Storage Access API. However, the benefits of a more reliable document.hasStorageAccess() are substantial. It will empower developers to build more robust and user-friendly web applications, ensuring that embedded content can function as intended while respecting user privacy.

So, what do you guys think? Have you run into this issue yourselves? Let's keep the conversation going and work towards a better Storage Access API for everyone!