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---
name: x-content-type
description: "Use when auditing HTTP response headers on any web server or CDN for security hardening."
metadata:
category: security
priority: high
difficulty: beginner
estimatedTime: "5"
source: frontendchecklist.io
url: https://frontendchecklist.io/rules/security/x-content-type
---
# 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.
---
For full implementation details, code examples, and framework-specific guidance,
see `references/rule.md`.
Rule page: https://frontendchecklist.io/rules/security/x-content-type
