# X Content Type

> Use when auditing HTTP response headers on any web server or CDN for security hardening.

- Skill: `thedaviddias/x-content-type` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add thedaviddias/x-content-type`
- Raw SKILL.md: https://api.skillmd.com/api/skills/thedaviddias/x-content-type/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- Author: thedaviddias (https://skillmd.com/u/thedaviddias)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/thedaviddias/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/en/rules/security/x-content-type