When processing untrusted input, servers are vulnerable to arbitrary code execution due to a critical flaw in next-mdx-remote, a popular TypeScript library for rendering MDX content in React applications This article explores attackers insert javascript. . After being identified by Sejong University researchers, the vulnerability—tracked as CVE-2026-0969—was revealed by HashiCorp on February 11, 2026, through security bulletin HCSEC-2026-01.

By failing to properly sanitize MDX content with enabled JavaScript expressions, the serialize function in versions 4.3.0 through 5.0.0 makes it possible for attackers to insert and run malicious code during server-side rendering (SSR). Insecure management of dynamic MDX compilation in client or server environments is the source of the vulnerability. Attackers can insert JavaScript payloads that evade sanitization when applications allow user-supplied MDX, which is typical in blogs, content management systems, and documentation platforms.

In order to accomplish remote code execution (RCE), these payloads make use of globals like eval, Function, process, or require. This could result in data exfiltration, lateral movement in cloud deployments, or complete server compromise. Echoing strategies seen in recent React ecosystem exploits, HashiCorp stressed that the risk is increased in SSR scenarios, where untrusted content is compiled directly on production servers.

The problem was fixed by HashiCorp in next-mdx-remote version 6.0.0, which included ground-breaking modifications for improved security. The update completely disables JavaScript expressions and stops the majority of RCE vectors by setting blockJS to true by default in both the serialize and compileMDX functions. Developers can disable blockJS: false for legacy use cases that need expressions, but doing so activates blockDangerousJS, which is also enabled by default and blocks high-risk operations using best-effort.

Configurations must be audited by organizations because incorrect configurations may reintroduce exposure. CVE Number Versions Affected by the Description Patch Version CVE-2026-0969 for CVSS Score When processing untrusted MDX content with JavaScript expressions enabled 4.3.0 – 5.0.0 9.8 (Critical) 6.0.0, arbitrary code execution occurs because the serialize function is not sufficiently sanitized. There are immediate risks for impacted organizations, especially those using React/Next.js stacks to handle dynamic content.

Upgrades to 6.0.0 are recommended immediately, along with content security and input validation procedures. Make ZeroOwl your Google Preferred Source.