The HTTP Cross-Origin-Embedder-Policy (COEP) response header configures embedding cross-origin resources into the document.

Header type Response header
Forbidden header name no


Cross-Origin-Embedder-Policy: unsafe-none | require-corp | credentialless



This is the default value. Allows the document to fetch cross-origin resources without giving explicit permission through the CORS protocol or the Cross-Origin-Resource-Policy header.


A document can only load resources from the same origin, or resources explicitly marked as loadable from another origin. If a cross origin resource supports CORS, the crossorigin attribute or the Cross-Origin-Resource-Policy header must be used to load it without being blocked by COEP.


no-cors cross-origin requests are sent without credentials. In particular, it means Cookies are omitted from the request, and ignored from the response. The responses are allowed without an explicit permission via the Cross-Origin-Resource-Policy header. Navigate responses behave similarly as the require-corp mode: They require Cross-Origin-Resource-Policy response header.


Certain features depend on cross-origin isolation

You can only access certain features like SharedArrayBuffer objects or with unthrottled timers, if your document has a COEP header with a value of require-corp or credentialless set.

Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Opener-Policy: same-origin

See also the Cross-Origin-Opener-Policy header which you'll need to set as well.

To check if cross origin isolation has been successful, you can test against the Window.crossOriginIsolated property or the WorkerGlobalScope.crossOriginIsolated property available to window and worker contexts:

const myWorker = new Worker("worker.js");

if (crossOriginIsolated) {
  const buffer = new SharedArrayBuffer(16);
} else {
  const buffer = new ArrayBuffer(16);

Avoiding COEP blockage with CORS

If you enable COEP using require-corp and have a cross origin resource that needs to be loaded, it needs to support CORS and you need to explicitly mark the resource as loadable from another origin to avoid blockage from COEP. For example, you can use the crossorigin attribute for this image from a third-party site:

<img src="" crossorigin />

If CORS is not supported for some images, a COEP value of credentialless can be used as an alternative to load the image without any explicit opt-in from the cross-origin server, at the cost of requesting it without cookies.


HTML Standard
# coep

Browser compatibility

BCD tables only load in the browser

See also