Cross-Origin Resource Policy (CORP)

Cross-Origin Resource Policy is an opt-in mechanism that allows web applications to protect against certain cross-origin requests, such as those issued by the browser when resources are embedded using elements such as <script> and <img>. This serves as an additional layer of protection above and beyond the same-origin policy which can mitigate speculative side channel attacks as well as Cross-Site Script Inclusion attacks. Cross-Origin Resource Policy complements Cross-Origin Read Blocking (CORB), which is a mechanism to prevent some cross-origin reads by default.

The policy is only effective for no-cors requests, which are issued by default for CORS-safelisted methods/headers.

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.

History

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.

Against this backdrop, Chromium shipped a feature known as Cross-Origin Read Blocking. This mechanism automatically protects a limited set of cross-origin 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 using a set of heuristics and apply the protection anyway.

Cross-Origin Resource Policy, in contrast, is an opt-in response header mechanism available to web developers which can protect any resource. There is no need for the browser to perform MIME type sniffing. The concept was originally proposed in 2012 (as From-Origin) but resurrected in Q2 of 2018 and implemented in Safari and Chromium.

Usage

Web applications set a Cross-Origin Resource Policy via the Cross-Origin-Resource-Policy HTTP response header.

The header accepts one of two values:

same-site

Only requests that the browser recognises as from the same Site (i.e. registrable domain) will be allowed to read the resource.

same-origin

Only requests that the browser recognises as from the same origin (i.e. [scheme, host, port] combination) will be allowed to read the resource.

Cross-Origin-Resource-Policy: same-site | same-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.

Browser compatibility

The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Specifications

See also

• Cross-Origin-Resource-Policy HTTP Header
• Bugzilla bug 1459573