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.
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---
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/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.
---
For full implementation details, code examples, and framework-specific guidance,
see `references/rule.md`.
Rule page: https://frontendchecklist.io/rules/security/cross-origin-isolation
