safe-mdx

Render MDX in React without eval

Features

• Render MDX without eval on the server, so you can render MDX in Cloudflare Workers and Vercel Edge
• Works with React Server Components
• Supports custom MDX components

Why

The default MDX renderer uses eval (or new Function(code)) to render MDX components in the server. This is a security risk if the MdX code comes from untrusted sources and it's not allowed in some environments like Cloudflare Workers.

For example in an hypothetical platform similar to Notion, where users can write Markdown and publish it as a website, an user could be able to write MDX code that extracts secrets from the server in the SSR pass, using this library that is not possible. This is what happened with Mintlify platform in 2024.

Some use cases for this package are:

• Render MDX in Cloudflare Workers and Vercel Edge
• Safely render dynamically generated MDX code, like inside a ChatGPT like interface
• Render user generated MDX, like in a multi-tenant SaaS app

Install

npm i safe-mdx

Usage

import { SafeMdxRenderer } from 'safe-mdx'

const code = `
# Hello world

This is a paragraph

<Heading>Custom component</Heading>
`

export function Page() {
    return (
        <MdxRenderer
            code={code}
            components={{
                // You can pass your own components here
                Heading({ children }) {
                    return <h1>{children}</h1>
                },
                p({ children }) {
                    return <p style={{ color: 'black' }}>{children}</p>
                },
                blockquote({ children }) {
                    return (
                        <blockquote style={{ color: 'black' }}>
                            {children}
                        </blockquote>
                    )
                },
            }}
        />
    )
}

Change default MDX parser

If you want to use custom MDX plugins, you can pass your own MDX processed ast.

By default safe-mdx already has support for

• frontmatter
• gfm

import { SafeMdxRenderer } from 'safe-mdx'
import { remark } from 'remark'
import remarkMdx from 'remark-mdx'

const code = `
# Hello world

This is a paragraph

<Heading>Custom component</Heading>
`

const parser = remark().use(remarkMdx)

const mdast = parser.parse(code)

export function Page() {
    return <MdxRenderer code={code} mdast={mdast} />
}

Handling errors

safe-mdx ignores missing components or expressions, to show a message to the user in case of these errors you can use MdastToJsx directly

import { MdastToJsx } from 'safe-mdx'

export function Page() {
    const visitor = new MdastToJsx({ code, mdast, components })
    const jsx = visitor.run()

    if (visitor.errors.length) {
        // handle errors here, like showing a message to the user for missing components
    }

    return jsx
}

Security

safe-mdx is designed to avoid server-side evaluation of untrusted MDX input.

However, it's important to note that safe-mdx does not provide protection against client-side vulnerabilities, such as Cross-Site Scripting (XSS) or script injection attacks. While safe-mdx itself does not perform any evaluation or rendering of user-provided content, the rendering library or components used in conjunction with safe-mdx may introduce security risks if not properly configured or sanitized.

This is ok if you render your MDX in isolation from each tenant, for example on different subdomains, this way an XSS attack cannot affect all tenants. If instead you render the MDX from different tenants on the same domain, one tenant could steal cookies set from other customers.

Limitations

These features are not supported yet:

• expressions with dynamic values or values defined with export
• importing components or data from other files

To overcome these limitations you can define custom logic in your components and pass them to SafeMdxRenderer. This will also make your MDX files cleaner and easier to read.