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
<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.
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.
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.
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:
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.
- Only requests from the same origin (i.e. scheme + host + port) can read the resource.
- Requests from any origin (both same-site and cross-site) can read the resource.
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.
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