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

Last updated · Reviewed by a CISSP-certified security practitioner

To add X-Content-Type-Options on Vercel, set the header in vercel.json with the value nosniff. Stops browsers from guessing (sniffing) file types. Commit and redeploy; headers apply on the next deployment. 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 Vercel configuration?

Add this to vercel.json:

vercel.json
{
  "headers": [
    {
      "source": "/(.*)",
      "headers": [
        {
          "key": "X-Content-Type-Options",
          "value": "nosniff"
        }
      ]
    }
  ]
}

How do I add it step by step?

  1. Open vercel.json.
  2. Paste the X-Content-Type-Options configuration shown below.
  3. Commit and redeploy; headers apply on the next deployment.
  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.
  • vercel.json headers don't apply to Next.js middleware responses — set them in next.config.js for Next apps.
  • The source pattern "/(.*)" matches every path.

Quick reference

HeaderX-Content-Type-Options
Recommended valuenosniff
StackVercel
Wherevercel.json
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 Vercel?

vercel.json headers don't apply to Next.js middleware responses — set them in next.config.js for Next apps. 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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