harden-react-markdown

harden-react-markdown

A wrapper for react-markdown that ensures that untrusted markdown does not contain images from and links to unexpected origins.

This is particularly important for markdown returned from LLMs in AI agents which might have been subject to prompt injection.

Secure prefixes

This package validates URL prefixes and URL origins. Prefix allow-lists can be circumvented with open redirects, so make sure to make the prefixes are specific enough to avoid such attacks.

E.g. it is more secure to allow https://example.com/images/ than it is to allow all of https://example.com/ which may contain open redirects.

Additionally, URLs may contain path traversal like /../. This package does not resolve these. It is your responsibility that your web server does not allow such traversal.

Features

• 🔒 URL Filtering: Blocks links and images that don't match allowed URL prefixes
• 🔧 Drop-in Replacement: Works with any react-markdown compatible component

Installation

npm install harden-react-markdown react react-markdown
# or
yarn add harden-react-markdown react react-markdown
# or
pnpm add harden-react-markdown react react-markdown

Quick Start

import React from "react";
import ReactMarkdown from "react-markdown";
import hardenReactMarkdown from "harden-react-markdown";

// Create a hardened version of ReactMarkdown
const HardenedMarkdown = hardenReactMarkdown(ReactMarkdown);

function MyComponent() {
  const markdown = `
# My Document
[Safe Link](https://github.com/user/repo)
[Blocked Link](https://malicious-site.com)
![Safe Image](https://via.placeholder.com/150)
![Blocked Image](https://evil.com/tracker.gif)
  `;

  return (
    <HardenedMarkdown
      defaultOrigin="https://mysite.com"
      allowedLinkPrefixes={["https://github.com/", "https://docs."]}
      allowedImagePrefixes={["https://via.placeholder.com/", "/"]}
    >
      {markdown}
    </HardenedMarkdown>
  );
}

API

hardenReactMarkdown(MarkdownComponent)

Creates a hardened version of any react-markdown compatible component.

Parameters

• MarkdownComponent: A React component that accepts Options from react-markdown

Returns

A new component with enhanced security that accepts all original props plus:

Props

defaultOrigin?: string

• The origin to resolve relative URLs against
• Required when allowedLinkPrefixes or allowedImagePrefixes are provided
• Example: "https://mysite.com"

allowedLinkPrefixes?: string[]

• Array of URL prefixes that are allowed for links
• Links not matching these prefixes will be blocked and shown as [blocked]
• Use "*" to allow all URLs (disables filtering. However, javascript: and data: URLs are always disallowed)
• Default: [] (blocks all links)
• Example: ['https://github.com/', 'https://docs.example.com/'] or ['*']

allowedImagePrefixes?: string[]

• Array of URL prefixes that are allowed for images
• Images not matching these prefixes will be blocked and shown as placeholders
• Use "*" to allow all URLs (disables filtering. However, javascript: and data: URLs are always disallowed)
• Default: [] (blocks all images)
• Example: ['https://via.placeholder.com/', '/'] or ['*']

All other props are passed through to the wrapped markdown component.

Examples

Basic Usage with Default Blocking

const HardenedMarkdown = hardenReactMarkdown(ReactMarkdown);

// Blocks all external links and images by default
<HardenedMarkdown>{markdownContent}</HardenedMarkdown>;

Allow Specific Domains

<HardenedMarkdown
  defaultOrigin="https://mysite.com"
  allowedLinkPrefixes={[
    "https://github.com/",
    "https://docs.github.com/",
    "https://www.npmjs.com/",
  ]}
  allowedImagePrefixes={[
    "https://via.placeholder.com/",
    "https://images.unsplash.com/",
    "/", // Allow relative images
  ]}
>
  {markdownContent}
</HardenedMarkdown>

Relative URL Handling

<HardenedMarkdown
  defaultOrigin="https://mysite.com"
  allowedLinkPrefixes={["https://mysite.com/"]}
  allowedImagePrefixes={["https://mysite.com/"]}
>
  {`
  [Relative Link](/internal-page)
  ![Relative Image](/images/logo.png)
  `}
</HardenedMarkdown>

Allow All URLs (Wildcard)

<HardenedMarkdown allowedLinkPrefixes={["*"]} allowedImagePrefixes={["*"]}>
  {`
  [Any Link](https://anywhere.com/link)
  ![Any Image](https://untrusted-site.com/image.jpg)
  `}
</HardenedMarkdown>

Note: Using "*" disables URL filtering entirely. Only use this when you trust the markdown source.

Custom Components

const CustomMarkdown = (props) => (
  <div className="custom-wrapper">
    <ReactMarkdown {...props} />
  </div>
);

const HardenedCustomMarkdown = hardenReactMarkdown(CustomMarkdown);

<HardenedCustomMarkdown
  defaultOrigin="https://mysite.com"
  allowedLinkPrefixes={["https://trusted.com/"]}
>
  {markdownContent}
</HardenedCustomMarkdown>;

Security Features

URL Filtering

• Links: Filters href attributes in <a> elements
• Images: Filters src attributes in <img> elements
• Relative URLs: Properly resolves and validates relative URLs against defaultOrigin
• Path Traversal Protection: Normalizes URLs to prevent ../ attacks
• Wildcard Support: Use "*" prefix to disable filtering (only when markdown is trusted)
• Prefix Matching: Validates that URLs start with allowed prefixes and have matching origins

Blocked Content Handling

• Blocked Links: Rendered as plain text with [blocked] indicator
• Blocked Images: Rendered as placeholder text with image description
• User Feedback: Clear indication when content has been blocked for security

Attack Prevention

• XSS Prevention: Blocks javascript:, data:, vbscript: and other dangerous protocols
• Redirect Protection: Prevents unauthorized redirects to malicious sites
• Tracking Prevention: Blocks unauthorized image tracking pixels
• Domain Spoofing: Validates full URLs, not just domains

TypeScript Support

Full TypeScript support with strict type checking:

// Type-safe component creation
const HardenedMarkdown = hardenReactMarkdown(ReactMarkdown);

// Inferred prop types include both react-markdown Options and security options
type Props = Parameters<typeof HardenedMarkdown>[0];

// Works with custom markdown components
const CustomMarkdown = (props: Options & { customProp?: string }) => (
  <ReactMarkdown {...props} />
);

const HardenedCustom = hardenReactMarkdown(CustomMarkdown);
// Props now include customProp + security options

Testing

The package includes comprehensive tests covering:

• Basic markdown rendering
• URL filtering for links and images
• Relative URL handling
• Security bypass prevention
• Edge cases and malformed URLs
• TypeScript type safety

Run tests:

npm test

Contributing

• Fork the repository
• Create your feature branch (git checkout -b feature/amazing-feature)
• Commit your changes (git commit -m 'Add some amazing feature')
• Push to the branch (git push origin feature/amazing-feature)
• Open a Pull Request

License

MIT License - see the LICENSE file for details.

Security

If you discover a security vulnerability, please send an e-mail to [e-mail].