- What information might this feature expose to Web sites or other parties, and for what purposes is that exposure necessary?
The Storage Access API enables the removal of cross-site cookies. Specifically, it allows the authenticated embeds use case to continue to work. As such, the API provides a way for developers to re-gain access to cross-site cookies, albeit under further constraints.
A nested Document gains access to the same cookies it has as the active document of a top-level browsing context when it calls document.requestStorageAccess() and is returned a resolving Promise. With these cookies it can authenticate itself to the server and load user-specific information.
While this functionality comes with a risk of abuse by third parties for tracking purposes, it is an explicit goal of the API and a key to its design to not undermine the gains of cross-site cookie deprecation.
- Do features in your specification expose the minimum amount of information necessary to enable their intended uses?
Where possible, yes:
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Permission grants for storage access are double-keyed on a (site, site) basis, meaning that requesting documents will not be able to access cross-site cookies outside of the top-level site that they were granted access under.
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To a single requesting document, this feature allows access to all its cross-site cookies (if given permission), which may or may not exceed the minimum necessary amount of information that document needs. However, given the flexible nature of cookies it is very hard to determine which cookies a document needs and developer flexibility is an explicit goal of the API. Also, from a privacy perspective, passing any single high-entropy identifier such as a cookie across the site boundary is equivalent to full cross-site cookie access.
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For security reasons, this API applies a few restrictions to how much information is exposed to a site that is granted storage access:
- Each storage access grant applies only to the nested document that requested it. Other documents have to re-request access, to ensure explicit opt-in from endpoints and prevent attacks from embedders.
- This specification calls out that implementers should still follow SameSite rules when attaching cross-site cookies with storage access. This API does not intend to waive existing security protections.
- How do the features in your specification deal with personal information, personally-identifiable information (PII), or information derived from them?
As mentioned, the SAA enables sharing of information through cross-site cookies, but does not expand on that or deal directly with PII in any way.
- How do the features in your specification deal with sensitive information?
See above.
- Do the features in your specification introduce new state for an origin that persists across browsing sessions?
Yes, a new "storage-access" permission that is managed via the permissions API and is double-keyed on (site, site). This should make it impossible for sites to access the new state across different top-level contexts.
- Do the features in your specification expose information about the underlying platform to origins?
No
- Does this specification allow an origin to send data to the underlying platform?
No
- Do features in this specification enable access to device sensors?
No
- Do features in this specification enable new script execution/loading mechanisms?
No
- Do features in this specification allow an origin to access other devices?
No
- Do features in this specification allow an origin some measure of control over a user agent's native UI?
While showing UI for storage access prompts is left largely implementation-defined, this API can generally be expected to enable origins to spawn permission prompts detailing the top-level site and the embedded site in the UI.
We have added a number of anti-abuse, spam and annoyance protections as outlined in the security considerations of the spec.
- What temporary identifiers do the features in this specification create or expose to the web?
None
- How does this specification distinguish between behavior in first-party and third-party contexts?
This specification is meant to be used in third-party contexts, i.e. in cross-site contexts (see https://tess.oconnor.cx/2020/10/parties). While the JS API itself will be accessible in a same-site context, it will resolve successfully by default without setting a storage access permission, to inform the developer that storage should be accessible by virtue of being in a same-site context. As such, the storage-access permission is not meaningful for any storage restrictions in same-site contexts (whatever those may look like).
- How do the features in this specification work in the context of a browser’s Private Browsing or Incognito mode?
The specification currently makes no explicit recommendation, as preferences may differ between user agents. It can generally be expected that in most user agents cross-site cookies are disabled in Private Browsing contexts, which would make it a natural fit for SAA. However, exposing prompts to users in private browsing that request sharing of data between two sites may be viewed as intrusive. The API includes sufficient mechanisms for user agents to always deny storage access requests in private browsing.
- Does this specification have both "Security Considerations" and "Privacy Considerations" sections?
Yes
- Do features in your specification enable origins to downgrade default security protections?
To some extent, yes, albeit with strong controls to prevent accidental loss of protection. Deprecation of cross-site cookies prevents certain class of attacks as outlined in detail in this document.
We apply the security restrictions as mentioned in point 2), specifically requiring strict per-document opt-in through calling rSA(), to avoid this loss of security.
- How does your feature handle non-"fully active" documents?
It will reject calls to rSA and hSA with an exception, as detailed in the spec.
- What should this questionnaire have asked?