Cross-Origin Read Blocking (CORB) is a web platform security feature aimed at reducing the impact of speculative side-channel attacks such as Spectre. Unfortunately, blocking certain types of requests introduced a new type of XS-Leaks 1 that allows attackers to detect if CORB was enforced on one request, but wasn’t on another. Nevertheless, the introduced XS-Leaks are much less problematic than the issues actively protected by CORB (e.g. Spectre).
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When a webpage issues a request to a server (e.g. fetch, HTML tags), the server receives and processes this request. When received, the server decides whether the request should succeed (e.g. 200) or fail (e.g. 404) based on the provided context. When a response has an error status, an error event is fired by the browser for the page to handle. These errors also cover situations where the parser fails, for example when trying to embed HTML content as an image.
Leaking the Content-Type of a request would provide attackers with a new way of distinguishing two requests from each other.
typeMustMatch is a Boolean that reflects the typeMustMatch attribute of the object element. It ensures that a certain MIME type must be enforced when loading an object, by verifying if the Content-Type of the resource is the same as the one provided in the object. Unfortunately, this enforcement also allowed attackers to leak the Content-Type and Status Codes returned by a website 1.