Understanding CVE-2026-0969: Remote Code Execution in next-mdx-remote

What Is next-mdx-remote?

next-mdx-remote is a popular library in the Next.js ecosystem that enables developers to render MDX content.

MDX is essentially Markdown with the ability to embed JSX (React components). It’s powerful because it allows content like this:

# Welcome

Here is a chart:

<CustomChart data={stats} />

Instead of limiting content to plain Markdown, MDX allows JavaScript expressions and React components inside documents. That flexibility is exactly what made this vulnerability serious.

The library is commonly used in:

• Developer documentation sites
• Content platforms and blogs
• Headless CMS integrations
• Marketing sites built with Next.js

It has been widely adopted because it allows MDX content to live outside the codebase, such as in a CMS or database, and be rendered at runtime.

What Was the Vulnerability?

The vulnerability (CVE-2026-0969) stems from how the library handled JavaScript expressions embedded inside MDX during server-side processing.

In affected versions (4.3.0 through 5.0.0):

• JavaScript expressions were enabled by default.
• The serialize() function did not sufficiently restrict or sanitize certain embedded expressions.

If an application accepted untrusted MDX input, that input could execute arbitrary JavaScript on the server.

That means an attacker could potentially inject something like:

{require('child_process').execSync('malicious command')}

If processed in a vulnerable setup, this could result in full remote code execution.

RCE is considered one of the most severe classes of vulnerabilities because it can allow attackers to:

• Execute system commands
• Access environment variables
• Read or modify files
• Move laterally inside infrastructure

Who Is Actually at Risk?

Not every project using next-mdx-remote is automatically vulnerable.

The key risk factor is this:

Does your application process MDX content that comes from an untrusted or user-controlled source?

Higher-risk scenarios include:

• CMS platforms where authors can input MDX
• Multi-user blogging platforms
• SaaS tools that allow user-generated documentation
• Any system storing MDX in a database and rendering it server-side

Lower-risk scenarios include:

• Static MDX files bundled directly into the application
• Content fully controlled by trusted developers
• Builds that occur only at compile time with no runtime MDX processing

Still, even “trusted content” assumptions can break over time as systems evolve.

How It Was Fixed

The vulnerability was addressed in version 6.0.0.

Key changes include:

• JavaScript expressions are no longer enabled by default.
• Safer defaults were introduced.
• Additional safeguards were implemented to block dangerous JS execution paths.

The fix shifts the security posture from permissive to defensive.

Why This Matters Beyond This Library

This incident highlights a broader pattern in modern web development:

Content is becoming executable.

Markdown is no longer “just text.”
Rendering pipelines are increasingly powerful.

MDX blends content and code. That’s powerful for developers but risky when content originates outside a fully trusted environment.

The vulnerability is not just about a single package. It’s about understanding that:

• Rendering engines are execution environments.
• Any time code is embedded in content, security boundaries must be clearly defined.
• Default configurations matter more than most teams realize.

What Developers Should Do

If you use next-mdx-remote:

• Upgrade to 6.0.0 or later immediately.
• Audit whether you process user-submitted MDX.
• Avoid enabling arbitrary JavaScript expressions unless absolutely necessary.
• Apply principle of least privilege to server environments.
• Treat content rendering as a potential attack surface.

Even if you believe your content is trusted, verify the trust chain.

References