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

Last updated · Reviewed by a CISSP-certified security practitioner

To add Cross-Origin-Opener-Policy on IIS, set the header in web.config with the value same-origin. Isolates your window from pop-ups and openers on other sites. Save web.config; IIS applies it on the next request. Recycle the app pool if needed. 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 IIS configuration?

Add this to web.config:

web.config
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="Cross-Origin-Opener-Policy" value="same-origin" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</configuration>

How do I add it step by step?

  1. Open web.config.
  2. Paste the Cross-Origin-Opener-Policy configuration shown below.
  3. Save web.config; IIS applies it on the next request. Recycle the app pool if needed.
  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.
  • Headers in web.config apply to every response from that site.
  • Removing the Server header requires removeServerHeader (IIS 10+) or URL Rewrite.

Quick reference

HeaderCross-Origin-Opener-Policy
Recommended valuesame-origin
StackIIS
Whereweb.config
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 IIS?

Headers in web.config apply to every response from that site. 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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