Set X-Content-Type-Options: nosniff

Browsers that MIME-sniff can be tricked into executing malicious JavaScript uploaded as an image — even if the server sends Content-Type: image/png. nosniff forces the browser to honor the declared type.

Quick Reference

• Set X-Content-Type-Options: nosniff on all responses — the only valid value is nosniff
• Without this header, browsers may execute a JavaScript file disguised as an image if the server serves it with the wrong MIME type
• This header is required by OWASP's security hardening checklist and the Fetch specification
• Pair with correct Content-Type headers on all responses for defense in depth
• Takes 5 minutes to configure and has no compatibility issues

Check

Check whether the server sends an X-Content-Type-Options: nosniff header on responses. Verify the header is present on HTML pages, scripts, stylesheets, and API responses.

Fix

Add X-Content-Type-Options: nosniff to all HTTP responses. Configure it at the web server level (Nginx, Apache) or in your application framework, and verify with curl -I https://example.com.

Explain

Explain what MIME type sniffing is, how it can be exploited to execute malicious files, and how X-Content-Type-Options: nosniff prevents this attack.

Code Review

Review server config, headers, forms, and integration points related to Set X-Content-Type-Options: nosniff. Flag exact responses, cookies, or browser behaviors that violate the rule, and verify them against the effective production-like response.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/en/rules/security/x-content-type