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How to add Cross-Origin-Opener-Policy on Netlify

Last updated · Reviewed by a CISSP-certified security practitioner

To add Cross-Origin-Opener-Policy on Netlify, set the header in _headers (in publish directory) with the value same-origin. Isolates your window from pop-ups and openers on other sites. Redeploy; check Deploy summary → Headers rules processed. Then confirm it with curl -I or a free FixSecurityHeaders scan.

What does Cross-Origin-Opener-Policy do?

COOP puts your page in its own browsing context group so cross-origin windows can't get a handle to it.

Risk if missing: Cross-window attacks (XS-Leaks, tabnabbing) and Spectre-style side channels.

What is the Netlify configuration?

Add this to _headers (in publish directory):

_headers (in publish directory)
/*
  Cross-Origin-Opener-Policy: same-origin

How do I add it step by step?

  1. Open _headers (in publish directory).
  2. Paste the Cross-Origin-Opener-Policy configuration shown below.
  3. Redeploy; check Deploy summary → Headers rules processed.
  4. Run curl -I https://yourdomain.com and look for the cross-origin-opener-policy line.
  5. Scan your site on FixSecurityHeaders to confirm the grade improved.

How do I verify it worked?

terminal
curl -sI https://yourdomain.com | grep -i "cross-origin-opener-policy"
# expected:
# cross-origin-opener-policy: same-origin

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What are the common mistakes?

  • OAuth or payment pop-ups that talk back via window.opener may need same-origin-allow-popups.
  • The _headers file must end up in your publish directory (e.g. public/).
  • Indent header lines under the path pattern.

Quick reference

HeaderCross-Origin-Opener-Policy
Recommended valuesame-origin
StackNetlify
Where_headers (in publish directory)
ComplianceOWASP ASVS V14.4

FAQ

What value should I use for Cross-Origin-Opener-Policy?

A safe default is same-origin. OAuth or payment pop-ups that talk back via window.opener may need same-origin-allow-popups.

Why isn't Cross-Origin-Opener-Policy showing up on Netlify?

The _headers file must end up in your publish directory (e.g. public/). Also clear any CDN or page cache, and check you edited the config that actually serves your domain.

Does Cross-Origin-Opener-Policy affect SEO or performance?

No measurable performance cost — it's a few bytes per response. It doesn't affect rankings directly, but a misconfigured policy can break page resources, so test before enforcing.

Which compliance frameworks reference Cross-Origin-Opener-Policy?

OWASP ASVS V14.4.

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