Cross-Origin Read Blocking

Cross-Origin Read Blocking

October 1, 2020
Category Defense

Cross-Origin Read Blocking (CORB) is a security mechanism that prevents attackers from loading certain cross-origin resources 1. This protection was created to defend against speculative side-channel attacks such as Spectre that allow attackers to read the memory of the process that both cross-site pages (e.g. and were embedded into. CORB aims to prevent attackers from loading certain sensitive cross-origin resources into an attacker-controlled process. For example, if an attacker tries to load cross-origin HTML, XML, or JSON into an img tag, CORB prevents this from happening. With CORB, the scenario is treated as though the server returned no data.

To classify resources, CORB uses the Content-Type header, the nosniff header, and a variety of other heuristics.


Cross-Origin Resource Policy (CORP) is an opt-in protection which enforces and extends CORB.

When using CORB, be aware of the following facts:

  • Currently, only Chromium-based browsers support CORB.
  • CORB does not protect against navigational requests. This means that in browsers that do not support out-of-process iframes, a CORB-protected resource may still end up in another origin’s process if framing protections are not used.
  • CORB introduces a new XS-Leak technique since attackers may be able to observe the results of CORB. This can lead to a variety of information leaks. However, in most cases, these information leaks have a lower impact than the data that could be leaked via speculative execution attacks.

References #

  1. Cross-Origin Read Blocking for Web Developers, link ↩︎