The HTTP Cross-Origin-Opener-Policy (COOP) response header allows you to ensure a top-level document does not share a browsing context group with cross-origin documents.

COOP will process-isolate your document and potential attackers can't access your global object if they were to open it in a popup, preventing a set of cross-origin attacks dubbed XS-Leaks.

If a cross-origin document with COOP is opened in a new window, the opening document will not have a reference to it, and the window.opener property of the new window will be null. This allows you to have more control over references to a window than rel=noopener, which only affects outgoing navigations.

Header type Response header
Forbidden header name no


Cross-Origin-Opener-Policy: unsafe-none
Cross-Origin-Opener-Policy: same-origin-allow-popups
Cross-Origin-Opener-Policy: same-origin



This is the default value. Allows the document to be added to its opener's browsing context group unless the opener itself has a COOP of same-origin or same-origin-allow-popups.


Retains references to newly opened windows or tabs that either don't set COOP or that opt out of isolation by setting a COOP of unsafe-none.


Isolates the browsing context exclusively to same-origin documents. Cross-origin documents are not loaded in the same browsing context.


Certain features depend on cross-origin isolation

Certain features like SharedArrayBuffer objects or with unthrottled timers are only available if your document has a COOP header with the value same-origin set.

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

See also the Cross-Origin-Embedder-Policy header which you'll need to set to require-corp or credentialless 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);


HTML Standard
# the-coop-headers

Browser compatibility

BCD tables only load in the browser

See also