COOP: cross origin opener policy

COEP: Cross origin embedder policy

Most of the articles on the web, related to COOP / COEP, point to the fact that by enabling COOP / COEP , your web page can use the sharedArrayBuffer and some other precision timer related functions.

I understand that there is a bigger concept of cross origin isolation which is connected to this, but in the current implementation, are there any other benefits of enabling COOP / COEP apart from allowing the web page to use these functionality ?

These are the things I already understand:

  • Cross origin isolation prevents against attacks like spectre which use the same process memory by using precision timers.
  • COOP: same-origin would mean that no other cross origin window would have a reference to your window, the parent window that opened your page, or the child window that your page opens. This seems like it would impact cross window communication with postMessage
  • COEP: require-corp would mean that any cross origin resources like images (which are generally exemptron CORS) would need to opt in, so that your page is allowed to load them

I am unable to understand what are the bigger implications of cross origin isolation. are all of the above side effects of cross origin isolation or are they intended effects ?


1 Answer 1


Your understanding is correct, but COOP/COEP are only partial solutions.

In a post-Spectre world, we want to avoid sharing a browser process with a potential attacker.

Sometimes, multiple documents from different sites can end up sharing a process in Chrome. This can happen when one has opened the other using window.open, or <a href="..." target="_blank">, or iframes. If a website contains user-specific data, there is a chance that another site could use these new vulnerabilities to read that user data.

Chrome Developers – Meltdown/Spectre

COOP: same-origin prevents other origins to share a browsing context group with your window through window.open or target="_blank" links without rel="noopener", regardless if they're in a parent or a child situation.

The Cross-Origin-Opener-Policy header provides a way for a document to isolate itself from cross-origin windows opened through window.open() or a link with target="_blank" without rel="noopener". As a result, any cross-origin opener of the document will have no reference to it and will not be able to interact with it.

web.dev – Security headers quick reference

An interesting thing to note though is that COOP doesn't affect iframes:

Because COOP is defined in terms of browsing context groups, it doesn't apply to iframes; the browser ignores COOP on documents which aren't top-level, and allows iframes to access their cross-origin ancestors even if these ancestors set COOP.


With this COOP the browser can ensure that, with the exception of frames, no cross-origin documents are present in the same browsing context group / process as the current document.

COOP and COEP explaned

On the COOP page on XS-Leaks Wiki they also highlight the fact it's a complement to framing protection:

Getting access to a website’s window object is a common prerequisite for different XS-Leak techniques. Framing Protections can ensure that an attacker cannot use iframes to access the window object, but this does not stop an attacker from accessing the window object from an opened window through window.open(url) or window.opener references.

XS Leaks Wiki – Cross-Origin-Opener-Policy

This is why we also need to set X-Frame-Options: SAMEORIGIN (or DENY). Even if you use COEP: require-corp, COOP: same-orgin and CORP: same-origin, you can still be embedded in an iframe if you don't set X-Frame-Options. You can test this in this demo. Not setting X-Frame-Options could leave you vulnerable to Spectre-like attacks in browsers that don't support out-of-process iframes.

As you pointed out, COEP only prevents you to include cross-origin resources that didn't explicitly opt in to it.

Cross-Origin-Opener-Policy: same-origin protects the origin from attackers
Cross-Origin-Embedder-Policy: require-corp protects victims from the origin

— InfoQ – COOP and COEP

This is why APIs that expose high precision timers are only available when COEP is enabled. Because you can perform Spectre-like attacks with those APIs, it makes sure that you can only attack resources that you have explicit permission to embed, limiting the attack surface quite a lot.

If you don't need those APIs, there's little benefit to enable COEP. If an attacker includes arbitrary resources on your site (for example through an XSS), COEP could prevent those resources to load, but in practice an attacker would likely want to include resources that they control, and where they can set CORP: cross-origin anyways.

To sum up, here's what both the W3C and Chrome recommend to mitigate Spectre-like attacks:

  1. Decide when (not!) to respond to requests by examining incoming headers, paying special attention to the Origin header on the one hand, and various Sec-Fetch- prefixed headers on the other.
  2. Restrict attackers' ability to load your data as a subresource by setting a cross-origin resource policy (CORP) of same-origin (opening up to same-site or cross-origin only when necessary).
  3. Restrict attackers' ability to frame your data as a document by opt-ing into framing protections via X-Frame-Options: SAMEORIGIN or CSP's more granular frame-ancestors directive (frame-ancestors 'self' https://trusted.embedder, for example).
  4. Restrict attackers' ability to obtain a handle to your window by setting a cross-origin opener policy (COOP). In the best case, you can default to a restrictive same-origin value, opening up to same-origin-allow-popups or unsafe-none only if necessary.
  5. Prevent MIME-type confusion attacks and increase the robustness of passive defenses like cross-origin read blocking (CORB) / opaque response blocking (ORB) by setting correct Content-Type headers, and globally asserting X-Content-Type-Options: nosniff.

W3C – Post-Spectre Web Development
Chromium Blog - Mitigating Side-Channel Attacks

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .