Comment by mmsc

And? Google uses Chrome to retrieve data about the user.

Every Chromium-based browser has 'hidden' APIs only accessible on certain domains. That's how the custom (read: closed source) extensions work. "Component extensions" are used to interact with them normally: https://chromium.googlesource.com/chromium/src/+/main/extens...

See https://blogs.opera.com/security/2021/09/8000-bug-bounty-hig... and https://blogs.opera.com/security/2021/09/bug-bounty-guest-po... for examples of when there are vulnerabilities in those extensions, and how they can be abused for remote code execution.

Any whitelisted domains for these APIs cannot be written to using user-installed extensions, in order for a malicious extension to not be able to inject a script and execute the special API.

At Opera, we previously tried attacking the underlying implementation about how these 'hidden' APIs are accessible. Although we found a lot of Opera-specific issues, the Chromium logic seems sound and a "bypass" for other websites accessing the API is unlikely. It also seems that the developer here was just a bit overzealous in allowing this API to be accessed from all google.com subdomains.