Cross-origin isolation enables a web page to use powerful features such as
SharedArrayBuffer. This article explains how to enable cross-origin
isolation on your website.
 Eiji Kitamura [X](https://twitter.com/agektmr) [GitHub](https://github.com/agektmr) [Mastodon](https://infosec.exchange/@agektmr) [Bluesky](https://bsky.app/profile/agektmr.com)
This guide shows you how to enable cross-origin isolation. Cross-origin
isolation is required if you want to use
[`SharedArrayBuffer`](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer),
[`performance.measureUserAgentSpecificMemory()`](https://web.dev/articles/monitor-total-page-memory-usage),
or [high resolution timer with better precision](https://developer.chrome.com/blog/cross-origin-isolated-hr-timers).
If you intend to enable cross-origin isolation, evaluate the impact this will
have on other cross-origin resources on your website, such as ad placements.
## Determine where in your website `SharedArrayBuffer` is used
Starting in Chrome 92, functionalities that use `SharedArrayBuffer`
will no longer work without cross-origin isolation. If you landed on this page
due to a `SharedArrayBuffer` deprecation message, it's likely either
your website or one of the resources embedded on it is using `SharedArrayBuffer`.
To ensure nothing breaks on your website due to deprecation, start by
identifying where it's used.
> [!NOTE]
> **Objective:**
>
> - Turn on cross-origin isolation to keep using `SharedArrayBuffer`.
> - If you rely on third-party code that uses `SharedArrayBuffer`, notify the third-party provider to take action.
If you are not sure where in your site a `SharedArrayBuffer` is used, there are two ways find out:
- Using Chrome DevTools
- (Advanced) Using Deprecation Reporting
If you already know where you are using `SharedArrayBuffer`, skip to [Analyze the impact of cross-origin isolation](https://web.dev/articles/cross-origin-isolation-guide#analysis).
### Using Chrome DevTools
[Chrome DevTools](https://developer.chrome.com/docs/devtools/open)
allow developers to inspect websites.
1. [Open the Chrome
DevTools](https://developer.chrome.com/docs/devtools/open) on the page you suspect might be using `SharedArrayBuffer`.
2. Select the **Console** panel.
3. If the page is using `SharedArrayBuffer`, the following message will show up:
```text
[Deprecation] SharedArrayBuffer will require cross-origin isolation as of M92, around May 2021. See for more details. common-bundle.js:535
```
4. The filename and the line number at the end of the message (for example, `common-bundle.js:535`) indicate where the `SharedArrayBuffer` is coming from. If it's a third-party library, contact the developer to fix the issue. If it's implemented as part of your website, follow the guide below to enable cross-origin isolation.
 DevTools Console warning when SharedArrayBuffer is used without cross-origin isolation.
### (Advanced) Using Deprecation Reporting
Some browsers have [a reporting functionality of deprecating
APIs](https://wicg.github.io/deprecation-reporting/) to a specified endpoint.
1. [Set up a deprecation report server and get the reporting
URL](https://web.dev/articles/coop-coep#observe_issues_using_the_reporting_api). You can achieve this by either using a public service or building one yourself.
2. Using the URL, set the following HTTP header to pages that are potentially serving `SharedArrayBuffer`.
```console
Report-To: {"group":"default","max_age":86400,"endpoints":[{"url":"THE_DEPRECATION_ENDPOINT_URL"}]}
```
3. Once the header starts to propagate, the endpoint you registered to should start collecting deprecation reports.
See [an example implementation](https://cross-origin-isolation.glitch.me).
## Analyze the impact of cross-origin isolation
Wouldn't it be great if you could assess the impact that enabling cross-origin
isolation would have on your site without actually breaking anything? The
[`Cross-Origin-Opener-Policy-Report-Only`](https://developer.mozilla.org/docs/Web/HTTP/Headers/Cross-Origin-Opener-Policy) and
[`Cross-Origin-Embedder-Policy-Report-Only`](https://developer.mozilla.org/docs/Web/HTTP/Headers/Cross-Origin-Embedder-Policy)
HTTP headers allow you to do just that.
1. Set [`Cross-Origin-Opener-Policy-Report-Only:
same-origin`](https://web.dev/articles/coop-coep#1_set_the_cross-origin-opener-policy_same-origin_header_on_the_top-level_document) on your top-level document. As the name indicates, this header only sends reports about the impact that `COOP: same-origin` **would** have on your site---it won't actually disable communication with popup windows.
2. Set up reporting and configure a web server to receive and save the reports.
3. Set [`Cross-Origin-Embedder-Policy-Report-Only:
require-corp`](https://web.dev/articles/coop-coep#3_use_the_coep_report-only_http_header_to_assess_embedded_resources) on your top-level document. Again, this header lets you see the impact of enabling `COEP: require-corp` without actually affecting your site's functioning yet. You can configure this header to send reports to the same reporting server that you set up in the previous step.
> [!NOTE]
> **Note:** You can also [enable the **Domain** column](https://developer.chrome.com/docs/devtools/network#information) in Chrome DevTools **Network** panel to get a general view of which resources would be impacted.
> [!CAUTION]
> **Caution:** Enabling cross-origin isolation will block the loading of cross-origin resources that you don't explicitly opt-in, and it will prevent your top-level document from being able to communicate with popup windows. We've been exploring ways to deploy `Cross-Origin-Resource-Policy` at scale, as cross-origin isolation requires all subresources to explicitly opt-in.
> And we have come up with the idea of going in the opposite direction: [a new COEP "credentialless" mode](https://developer.chrome.com/blog/coep-credentialless-origin-trial) that allows loading resources without the CORP header by stripping all their credentials. We hope this will lighten your burden of making sure the subresources are sending the `Cross-Origin-Resource-Policy` header.
> Though `credentialless` mode is available on Chrome from version 96, it's not supported by any other browsers yet, this may cause some developers find it challenging to deploy COOP or COEP at this stage.
> Also, it's known that the `Cross-Origin-Opener-Policy: same-origin` header will break integrations that require cross-origin window interactions such as OAuth and payments. To mitigate this problem, we are exploring [relaxing the condition](https://github.com/whatwg/html/issues/6364) to enable cross-origin isolation to `Cross-Origin-Opener-Policy: same-origin-allow-popups`.
> This way the communication with the window opened by itself will be possible. If you want to enable cross-origin isolation but are blocked by these challenges, we recommend [registering for an origin trial](https://developer.chrome.com/blog/enabling-shared-array-buffer#origin-trial) and waiting until the new modes are available. We are not planning to terminate the origin trial until these new modes are available.
## Mitigate the impact of cross-origin isolation
After you have determined which resources will be affected by cross-origin
isolation, here are general guidelines for how you actually opt-in those
cross-origin resources:
1. On cross-origin resources such as images, scripts, stylesheets, iframes, and others, set the [`Cross-Origin-Resource-Policy:cross-origin`](https://resourcepolicy.fyi/#cross-origin) header. On same-site resources, set [`Cross-Origin-Resource-Policy:same-site`](https://resourcepolicy.fyi/#same-origin) header.
2. For resources loadable using [CORS](https://web.dev/articles/cross-origin-resource-sharing), make sure it is enabled, by setting the `crossorigin` attribute in its HTML tag (for example, `
`). For JavaScript fetch request, make sure `request.mode` is set to `cors`.
3. If you want to use powerful features such as `SharedArrayBuffer` inside a loaded iframe, append `allow="cross-origin-isolated"` to the `