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. Cross-Origin Resource Policy complements Cross-Origin Read Blocking (CORB), which is a mechanism to prevent some cross-origin reads by default.
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 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.
Specifications
Specification |
---|
Fetch Standard # cross-origin-resource-policy-header |
Browser compatibility
BCD tables only load in the browser
See also
Cross-Origin-Resource-Policy
HTTP Header