Cross-Origin Resource Policy (CORP)

Cross-Origin Resource Policy is a policy set by the Cross-Origin-Resource-Policy HTTP header that lets web sites and applications opt in to protection against certain requests from other origins (such as those issued with elements like <script> and <img>), to mitigate speculative side-channel attacks, like Spectre, as well as Cross-Site Script Inclusion attacks.

CORP is an additional layer of protection beyond the default same-origin policy. Cross-Origin Resource Policy complements Cross-Origin Read Blocking (CORB), which is a mechanism to prevent some cross-origin reads by default.

As this policy is expressed via a response header, the actual request is not prevented—rather, the browser prevents the result from being leaked by stripping the response body.

Web applications set a Cross-Origin Resource Policy via the Cross-Origin-Resource-Policy HTTP response header, which accepts one of three values:

same-site

Only requests from the same Site can read the resource.

Warning: This is less secure than an origin. The algorithm for checking if two origins are same site is defined in the HTML standard and involves checking the registrable domain.

same-origin

Only requests from the same origin (i.e. scheme + host + port) can read the resource.

cross-origin

Requests from any origin (both same-site and cross-site) can read the resource. This is useful when COEP is used (see below).

Cross-Origin-Resource-Policy: same-site | same-origin | cross-origin

During a cross-origin resource policy check, if the header is set, the browser will deny no-cors requests issued from a different origin/site.

The Cross-Origin-Embedder-Policy HTTP response header, when used upon a document, can be used to require subresources to either be same-origin with the document, or come with a Cross-Origin-Resource-Policy HTTP response header to indicate they are okay with being embedded. This is why the cross-origin value exists.

The concept was originally proposed in 2012 (as From-Origin), but resurrected in Q2 of 2018 and implemented in Safari and Chromium.

In early 2018, two side-channel hardware vulnerabilities known as Meltdown and Spectre were disclosed. These vulnerabilities allowed sensitive data disclosure due to a race condition which arose as part of speculative execution functionality, designed to improve performance.

In response, Chromium shipped Cross-Origin Read Blocking, which automatically protects certain resources (of Content-Type HTML, JSON and XML) against cross-origin reads. If the application does not serve a no-sniff directive, Chromium will attempt to guess the Content-Type and apply the protection anyway.

Cross-Origin-Resource-Policy is an opt-in response header which can protect any resource; there is no need for browsers to sniff MIME types.

• Cross-Origin-Resource-Policy HTTP Header