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How to add X-Content-Type-Options on IIS

Last updated · Reviewed by a CISSP-certified security practitioner

To add X-Content-Type-Options on IIS, set the header in web.config with the value nosniff. Stops browsers from guessing (sniffing) file types. 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 X-Content-Type-Options do?

Setting nosniff makes the browser trust the Content-Type you send instead of guessing, so an uploaded text file can't be executed as script.

Risk if missing: MIME sniffing can turn user uploads or JSON responses into executable scripts or styles.

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="X-Content-Type-Options" value="nosniff" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</configuration>

How do I add it step by step?

  1. Open web.config.
  2. Paste the X-Content-Type-Options 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 x-content-type-options 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 "x-content-type-options"
# expected:
# x-content-type-options: nosniff

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What are the common mistakes?

  • Make sure your server sends correct Content-Type headers, or some assets will stop loading.
  • Headers in web.config apply to every response from that site.
  • Removing the Server header requires removeServerHeader (IIS 10+) or URL Rewrite.

Quick reference

HeaderX-Content-Type-Options
Recommended valuenosniff
StackIIS
Whereweb.config
ComplianceOWASP ASVS V14.4.4, SOC 2 CC6.8

FAQ

What value should I use for X-Content-Type-Options?

A safe default is nosniff. Make sure your server sends correct Content-Type headers, or some assets will stop loading.

Why isn't X-Content-Type-Options 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 X-Content-Type-Options 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 X-Content-Type-Options?

OWASP ASVS V14.4.4, SOC 2 CC6.8.

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