Fetch Metadata

Introduces a new HTTP request header that sends additional metadata about a request's provenance (is it cross-site, is it triggered from <img>, etc.) to the server to allow it to make security decisions which might mitigate some kinds of attacks based on timing the server's response (xsleaks and others).

Comments

For example, it's unlikely that a bank's "Transfer all money" endpoint would be referenced from an img tag, and likewise unlikely that evil.com is going to be making any legitimate requests whatsoever. Ideally, the server could reject these requests a priori rather than deliver them to the application backend.