Cross-Origin Resource Policy (CORP)

Cross-Origin Resource Policy is a policy set by the Cross-Origin-Resource-Policy HTTP header that lets websites 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.

[!NOTE] 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.

Usage

[!NOTE] Due to a bug in Chrome, setting Cross-Origin-Resource-Policy can break PDF rendering, preventing visitors from being able to read past the first page of some PDFs. Exercise caution using this header in a production environment.

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.

Relationship to cross-origin embedder policy (COEP)

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.

History

The concept was originally proposed in 2012 (as a From-Origin header), 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.

Cross-Origin Resource Policy was developed as a direct way for sites to block unwanted no-cors cross-origin requests. This is an effective defense against Spectre-like attacks, as the browser strips the body from given responses before an attacker can access them.

Specifications

Browser compatibility

See also

• Cross-Origin-Resource-Policy HTTP Header