Content-Type Sniffer Checker
Enter any Content-Type HTTP header value to check if it is vulnerable to MIME-type sniffing attacks. Optionally provide the X-Content-Type-Options header value to validate whether nosniff is properly configured. Our tool analyzes 20+ common MIME types with detailed risk assessments - all processing happens locally in your browser, no signup required.
Enter a Content-Type header value to check if it is vulnerable to MIME-type sniffing attacks. Optionally provide the X-Content-Type-Options header value to validate whether nosniff is properly configured.
Paste the full Content-Type header value, including any parameters like charset or boundary
Optional. Enter the X-Content-Type-Options header value if your response includes it
Enter a Content-Type header
Check if your Content-Type is vulnerable to MIME sniffing
All analysis is performed locally in your browser. No data is sent to any server.
Why Use Our Content-Type Sniffer Checker?
Instant MIME Type Analysis
Paste any Content-Type header value and instantly identify the MIME type, its category (HTML, Image, Script, etc.), parameters like charset and boundary, and whether it is one of 20+ common content types used on the web.
X-Content-Type-Options Validation
Optionally provide the X-Content-Type-Options header value to validate whether nosniff is properly configured. The tool instantly detects if the nosniff directive is present and correctly formatted to prevent MIME-type sniffing.
Sniffing Risk Assessment
Get a clear risk assessment - Secure, Low, Medium, or High - based on your Content-Type and X-Content-Type-Options configuration. Understand exactly how vulnerable your content is to MIME-type sniffing attacks.
Actionable Security Recommendations
Receive step-by-step recommendations tailored to your specific Content-Type and header configuration. Learn exactly what changes to make to protect your users from content-type sniffing vulnerabilities and XSS attacks.
Common Use Cases for Content-Type Sniffer Checker
Web Security Audit & Hardening
During security audits, check every response Content-Type header from your web application. Verify that X-Content-Type-Options: nosniff is configured globally and that no endpoints are vulnerable to MIME sniffing attacks that could lead to XSS.
Penetration Testing & Vulnerability Assessment
Identify MIME sniffing vulnerabilities during penetration testing. Check if file upload endpoints, CDN origins, or API responses are missing the nosniff header, allowing attackers to disguise malicious content as safe file types.
CDN & Reverse Proxy Configuration
Verify that your CDN, load balancer, or reverse proxy is correctly passing through or adding the X-Content-Type-Options header. Misconfigured proxies can strip security headers, leaving your origin responses vulnerable to sniffing.
Compliance & Regulatory Requirements
Meet security compliance standards (PCI DSS, OWASP Top 10, ISO 27001, HITRUST) that require proper security headers. The X-Content-Type-Options header is a critical control for information leakage prevention and content security.
File Upload Security Review
When users upload files to your application, verify that uploaded content is served with correct Content-Type headers and the nosniff directive. Prevent attackers from uploading disguised HTML/JavaScript files that could execute in other users' browsers.
API Response Header Verification
Check that your REST API, GraphQL endpoint, or webhook responses include proper Content-Type and X-Content-Type-Options headers. API consumers may be vulnerable to sniffing if responses use generic content types without protection.
Understanding MIME Type Sniffing
What Is MIME Type Sniffing?
MIME type sniffing (also called content sniffing) is a browser behavior where the browser ignores the Content-Type header declared by the server and instead inspects the actual content to guess what type of data it is. This was originally designed for backward compatibility with misconfigured servers, but it creates a security vulnerability: an attacker can upload a file that claims to be an image (Content-Type: image/png) but contains HTML with JavaScript. Without X-Content-Type-Options: nosniff, the browser might execute the content as HTML, enabling cross-site scripting (XSS).
How Our Content-Type Sniffer Checker Works
- Enter the Content-Type: Paste the Content-Type header value from your HTTP response. Include parameters like charset and boundary if present. The tool parses the main MIME type and all parameters.
- Optionally Add X-Content-Type-Options: Enter the X-Content-Type-Options header value if your response includes it. The tool checks if it is set to "nosniff" (the only valid value that provides protection).
- Risk Assessment: The tool analyzes the MIME type against a database of known types, their sniffing risk levels, and the presence of the nosniff directive. It generates a risk level (Secure, Low, Medium, High) with specific, actionable recommendations for improving your security posture.
Understanding Content-Type Parameters
- Charset: Specifies the character encoding of the content (e.g., charset=utf-8). This tells the browser how to decode the bytes into characters. Incorrect charset declarations can lead to encoding-based attacks.
- Boundary: Used in multipart Content-Types (multipart/form-data) to separate different parts of a message. The boundary parameter is a unique string that appears between each part of the multipart content.
- Media Type Structure: Content-Type follows the format type/subtype; parameter=value. The type is the top-level category (text, image, audio, video, application, multipart, message), and the subtype is the specific format (html, png, json, etc.).
- Syntax Validation: The tool parses the Content-Type header to extract and display each parameter. While basic syntax is checked, full RFC 7231 validation requires checking for proper quoting of parameter values and the correct use of the semicolon delimiter.
Privacy, Security & Availability
Our Content-Type Sniffer Checker processes everything locally in your browser. The header values you enter are never sent to our servers - all parsing, analysis, and recommendation generation happens client-side using pure JavaScript. The tool is 100% free with no signup, no account, and no usage limits. We maintain a comprehensive database of 20+ common MIME types with their sniffing risk characteristics, and the tool can analyze any Content-Type value including custom or vendor-specific types.
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Frequently Asked Questions About Content-Type Sniffer Checker
MIME type sniffing is a browser behaviour where the browser ignores the declared Content-Type header and inspects the actual content to determine its type. While designed to handle misconfigured servers, it creates a security risk: attackers can serve malicious content (like HTML with JavaScript) with a benign Content-Type (like image/png), and the browser may execute it as HTML, leading to XSS attacks.
The X-Content-Type-Options response header with the value "nosniff" tells the browser to strictly follow the declared Content-Type header and not perform MIME type sniffing. For script and stylesheet resources, it also blocks MIME type mismatches (e.g., blocking a script served as text/html). It is one of the most important security headers and should be set on every response.
On Nginx, add `add_header X-Content-Type-Options "nosniff" always;` to your configuration. On Apache, use `Header always set X-Content-Type-Options "nosniff"`. On IIS, it's set via the `httpProtocol/customHeaders` section in web.config. Most application frameworks (Express with Helmet, Django, Rails) include this header by default or can be configured to add it.
text/html, text/javascript, application/octet-stream, image/svg+xml, and text/xml are the most vulnerable. HTML content can be executed as web pages anywhere. JavaScript can be executed in the browser context. SVG files can contain embedded scripts. Generic binary (octet-stream) is particularly dangerous because browsers will try to sniff its actual type from the content bytes.
Setting X-Content-Type-Options: nosniff is essential, but it should be part of a broader security header strategy. Combine it with Content-Security-Policy to restrict which scripts and styles can execute, X-Frame-Options to prevent clickjacking, and proper Content-Type declarations on every response. Also ensure your server never serves one file type with an incorrect Content-Type header.
Without X-Content-Type-Options, browsers may perform content sniffing on responses. For example, if an attacker uploads a file named "image.png" that actually contains HTML with JavaScript, and your server serves it with Content-Type: image/png, a browser without nosniff protection may render it as HTML and execute the script, leading to a cross-site scripting (XSS) vulnerability.
Without the header, browsers decide whether to sniff based on their own heuristics and the Content-Type value. With "nosniff", browsers are explicitly instructed to trust the Content-Type header. This removes ambiguity and ensures predictable, secure behaviour. Setting the header to any value other than "nosniff" (e.g., "nosniff; charset=utf-8") is treated as invalid and provides no protection.
Yes! Our Content-Type Sniffer Checker is completely free with no signup, no account, and no usage limits. All analysis happens locally in your browser - your header values are never sent to any server.