cross-origin-isolation

name	cross-origin-isolation
description	Use when reviewing security-sensitive web apps, SharedArrayBuffer usage, worker-heavy apps, editors, or measurement features that require cross-origin isolation. Check both headers and real browser behavior.
metadata	{"category":"security","priority":"medium","difficulty":"advanced","estimatedTime":"30","source":"frontendchecklist.io","url":"https://frontendchecklist.io/en/rules/security/cross-origin-isolation"}

Use COOP, COEP, and CORP for cross-origin isolation when needed

Cross-origin isolation reduces opener-based attacks and XS-Leak risk, and it is required for powerful browser features such as SharedArrayBuffer in modern browsers. It also breaks integrations if deployed carelessly, so it needs a deliberate rollout rather than a copy-pasted header block.

Quick Reference

• Use Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy together when you need cross-origin isolation
• Audit every cross-origin script, iframe, image, worker, and font before enforcing COEP
• Serve embeddable resources with CORS or Cross-Origin-Resource-Policy as appropriate
• Verify self.crossOriginIsolated === true before depending on isolated-only features

Check

Review security headers and embedded resources for cross-origin isolation. Check whether the app needs COOP/COEP, whether resources are compatible with COEP, and whether the runtime actually becomes crossOriginIsolated.

Fix

Add COOP and COEP only after auditing dependencies, ensure subresources use CORP or CORS as required, and verify the final document becomes crossOriginIsolated in the browser.

Explain

Explain how COOP, COEP, and CORP work together, what they protect against, and why third-party embeds often block a successful rollout.

Code Review

Review server headers, third-party integrations, popups, workers, and asset responses related to Use COOP, COEP, and CORP for cross-origin isolation when needed. Flag exact responses or integrations that would block isolation or break once isolation is enforced.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/en/rules/security/cross-origin-isolation