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: nosniffon all responses — the only valid value isnosniff - 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-Typeheaders 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.
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