The correct JSON content type to use, and when each should be applied

The correct and standard MIME type for JSON is application/json. This should be used in all modern contexts when serving JSON data, including APIs, configuration files, and data interchange. While some older systems might use alternative types like text/json, application/json offers better security and broader browser compatibility, making it the recommended choice for new implementations.

Contents

• The Standard JSON Content Type
• Alternative JSON Content Types and Their Origins
• Security Considerations
• Browser Compatibility Factors
• Best Practices by Context
• Historical Evolution of JSON Content Types
• Future Trends

The Standard JSON Content Type

application/json is the officially registered media type for JSON (JavaScript Object Notation) as defined by IANA (Internet Assigned Numbers Authority). This MIME type was registered in 2006 and has since become the de facto standard for JSON data transmission on the web.

The JSON specification (RFC 8259) explicitly recommends using application/json for JSON text. This content type indicates that the body contains JSON data, which allows clients to correctly interpret and process the content without additional inspection.

When a server responds with application/json, it’s signaling that the content is structured data that should be parsed according to JSON syntax rules, rather than being treated as plain text or executable code. This distinction is crucial for both security and proper data handling.

Example of a proper HTTP response header:

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Content-Length: 123

{"name": "John Doe", "age": 30}

Alternative JSON Content Types and Their Origins

While application/json is the standard, several alternative MIME types have been used historically or in specific contexts:

text/json

This was an early alternative to application/json. The main difference between text/* and application/* media types is that text/* types imply a human-readable character set, typically with a default of ISO-8859-1, while application/* types require an explicit charset parameter if needed.

text/json is functionally equivalent to application/json but doesn’t follow the official registration. Some systems may still use it for backward compatibility with older parsers that didn’t recognize application/json.

JavaScript-related Content Types

Several content types associated with JavaScript have been used to deliver JSON data, primarily for historical reasons:

• application/x-javascript: This was a non-standard but widely used content type for JavaScript before application/javascript was officially registered.
• text/javascript: The standard content type for JavaScript files, though application/javascript is now preferred.
• text/x-javascript: A non-standard variant sometimes used to avoid conflicts with server configurations that might block certain content types.
• text/x-json: An unofficial content type that emerged before application/json was widely supported.

These JavaScript-related content types should generally not be used for pure JSON data, as they may cause browsers to attempt to execute JSON as JavaScript, creating potential security vulnerabilities.

Security Considerations

The choice of content type has significant security implications when working with JSON data:

Content-Type Sniffing and Security

Browsers employ content sniffing to determine the type of content when the Content-Type header is missing or incorrect. This can lead to security issues, particularly when using JavaScript-related MIME types for JSON.

When JSON is served with a JavaScript content type (like text/javascript), some browsers may attempt to execute it as JavaScript if it’s included in a <script> tag. This is dangerous because:

1. Cross-Site Scripting (XSS) Vulnerabilities: If the JSON data contains malicious code and is served with a JavaScript content type, it could be executed in the context of your website.

2. JSON Hijacking: In older browsers, JSON responses served with JavaScript content types were vulnerable to JSON hijacking attacks where attackers could extract sensitive data.

Protection Mechanisms

Using application/json helps mitigate these risks because:

1. Browsers won’t execute JSON as code: Modern browsers recognize application/json as data to be parsed, not code to be executed.

2. CORS (Cross-Origin Resource Sharing): When JSON APIs use application/json, proper CORS headers can be applied to control cross-origin access.

3. Content Security Policy (CSP): Modern security practices rely on correct Content-Type headers to implement effective CSPs.

Security Best Practice: Always use application/json for JSON data, never use JavaScript MIME types unless you intentionally want the content to be treated as executable JavaScript.

Browser Compatibility Factors

Support for application/json

Modern browsers have excellent support for application/json:

• All modern browsers (Chrome, Firefox, Safari, Edge) fully support application/json and correctly interpret it as JSON data.
• Even Internet Explorer 8+ supports application/json when used with proper XMLHttpRequest or Fetch API calls.

Browser Quirks and Legacy Issues

Some older browsers or specific contexts may have quirks:

• Internet Explorer 7 and earlier do not natively support application/json and may require polyfills or workarounds.
• Some older mobile browsers might have issues with charset handling when using application/json without explicit charset specification.
• In some contexts, particularly with older server-side frameworks, there might be legacy configurations that expect JSON with alternative content types.

Browser Content Sniffing Behavior

Different browsers handle content sniffing differently:

• Chrome and Firefox have become more strict about respecting declared Content-Type headers and are less likely to attempt to execute JSON served with JavaScript MIME types.
• Safari has historically been more aggressive in content sniffing, which is another reason to always use the correct application/json type.

Best Practices by Context

Web APIs

For REST APIs and web services:

• Always use application/json for JSON responses.
• Include the charset=utf-8 parameter for explicit character encoding specification.
• Set appropriate CORS headers when your API might be accessed from different origins.
• Consider using application/problem+json for error responses following RFC 7807.

Configuration Files

For JSON-based configuration files:

• Use application/json for configuration files that will be parsed by applications.
• For human-readable configuration that might be edited directly, text/json could be considered, though application/json is still preferred.
• For specific configuration formats like JSON Schema, consider application/schema+json.

JavaScript in Browsers

When working with JSON in browser contexts:

• When retrieving JSON via AJAX/Fetch, always expect and handle application/json responses.
• When dynamically creating <script> tags for JSONP (a now-discouraged technique), use text/javascript as the content type, but be aware of security implications.
• For modern cross-domain data retrieval, use CORS with application/json instead of JSONP.

Server Configuration

When configuring servers to serve JSON:

• Apache: Add AddType application/json .json to serve .json files with the correct type.
• Nginx: Include application/json in your types configuration.
• IIS: Configure MIME types to include application/json.

Implementation Tip: Test your JSON endpoints with different HTTP clients to verify that the Content-Type header is correctly set to application/json and that the response body is valid JSON.

Historical Evolution of JSON Content Types

The history of JSON content types reflects the evolution of web technologies:

• Early 2000s: JSON emerged as a lightweight alternative to XML. Initially, it was often served with text/plain or sometimes with JavaScript-related MIME types.
• 2006: application/json was officially registered with IANA.
• 2009-2010: As JSON APIs became popular, application/json gained adoption, though some legacy systems continued using alternative types.
• 2013-2015: With the rise of single-page applications and REST APIs, application/json became the undisputed standard.
• 2019-Present: Modern web standards fully embrace application/json as the correct content type for JSON data.

This evolution shows a clear trend toward standardization and away from the ad-hoc use of JavaScript-related content types for JSON data.

Future Trends

Several trends are shaping the future of JSON content type usage:

Increased Standardization

• New web specifications are increasingly mandating application/json for JSON content.
• Frameworks and libraries are defaulting to application/json with fewer configuration options for alternative types.

Enhanced Security Practices

• Future web security standards will likely place even more emphasis on correct Content-Type headers.
• Browsers may become stricter about enforcing content type boundaries, potentially executing fewer “sniffed” content types.

New JSON-Based Formats

• Emerging formats like JSON-LD, JSON Schema, and JSON API are building on JSON while introducing their own specific content types:
  • application/ld+json for JSON-LD
  • application/schema+json for JSON Schema
  • application/vnd.api+json for JSON API

Performance Considerations

• Future web performance optimization may focus on more efficient ways to serve structured data, potentially leading to new content types or compression techniques specifically for JSON.

Conclusion

Key Takeaways:

1. Standardization: application/json is the official, standardized MIME type for JSON data and should be used in all modern contexts.

2. Security: Using JavaScript-related content types for JSON creates potential security vulnerabilities and should be avoided unless there’s a specific, justified reason.

3. Compatibility: All modern browsers fully support application/json, making it safe to use without concerns about browser support.

4. Consistency: Consistently using application/json across your systems reduces confusion and potential integration issues.

Practical Recommendations:

• For new APIs and web services, default to application/json with explicit UTF-8 charset.
• When working with existing systems that use alternative content types, understand the context but plan to migrate to application/json when possible.
• Always validate JSON responses both structurally and by verifying the Content-Type header.
• Consider security implications before ever serving JSON with JavaScript-related MIME types.

Related Questions:

• What should I do if I’m forced to use a legacy system that requires JSON with a non-standard content type?

  • In such cases, you may need to use the required content type while implementing additional validation on the client side to ensure the content is actually JSON.

• Are there any legitimate use cases for serving JSON with a JavaScript content type today?

  • Generally no, except in very specific legacy scenarios or when implementing JSONP (though JSONP itself is discouraged in favor of CORS).

• How can I verify that my JSON endpoints are serving the correct content type?

  • Use browser developer tools to inspect response headers, or test with HTTP clients like curl, Postman, or HTTPie to verify the Content-Type header.

By following these guidelines, you’ll ensure your JSON implementations are secure, compatible, and aligned with current web standards.