CVE-2025-24981

An unsafe parsing logic of the URL from markdown can lead to arbitrary JavaScript code due to a bypass to the existing guards around the javascript: protocol scheme in the URL.

Details

The parsing logic implement at https://github.com/nuxt-modules/mdc/blob/main/src/runtime/parser/utils/props.ts#L16 maintains a deny-list approach to filtering potential malicious payload. It does so by matching protocol schemes like javascript: and others.

Specifically, this is the code from the mdc library's parser that is not secure enough:

export const unsafeLinkPrefix = [
  'javascript:',
  'data:text/html',
  'vbscript:',
  'data:text/javascript',
  'data:text/vbscript',
  'data:text/css',
  'data:text/plain',
  'data:text/xml'
]

export const validateProp = (attribute: string, value: string) => {
  if (attribute.startsWith('on')) {
    return false
  }

  if (attribute === 'href' || attribute === 'src') {
    return !unsafeLinkPrefix.some(prefix => value.toLowerCase().startsWith(prefix))
  }

  return true
}

These security guards can be bypassed by an adversarial that provides JavaScript URLs with HTML entities encoded via hex string.

PoC

The following URL payloads if provided to the markdown parsing library (such as through the usage of import { parseMarkdown } from '@nuxtjs/mdc/runtime';) will trigger the alert() dialog:

# ✅ This is correctly escaped by the parser

- XSS Attempt:

<a href="javascript:alert(1)"> this gets sanitizied, yay!</a>

# ❌ These are vulnerable and not escaped

- Bypass 1:

<a href="jav&#x09;ascript:alert('XSS');">Click Me 1</a>

- Bypass 2:

<a href="jav&#x0A;ascript:alert('XSS');">Click Me 2</a>

- Bypass 3:

<a href="jav&#10;ascript:alert('XSS');">Click Me 3</a>

Reference

You may infer the following write-up for more in-depth walkthrough of URL parsing problems and suggestions on how to securely address them: How to Parse URLs from Markdown to HTML Securely?

Users who consume this library and perform markdown parsing from unvalidated sources such as LLM generative text responses, user input and other untrusted sources could result in rendering vulnerable XSS anchor links.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2025-24981 has a CVSS score of 9.3 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.13.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-24981? CVE-2025-24981 is a critical-severity cross-site scripting (XSS) vulnerability in @nuxtjs/mdc (npm), affecting versions <= 0.13.2. It is fixed in 0.13.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2025-24981? CVE-2025-24981 has a CVSS score of 9.3 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of @nuxtjs/mdc are affected by CVE-2025-24981? @nuxtjs/mdc (npm) versions <= 0.13.2 is affected.
4. Is there a fix for CVE-2025-24981? Yes. CVE-2025-24981 is fixed in 0.13.3. Upgrade to this version or later.
5. Is CVE-2025-24981 exploitable, and should I be worried? Whether CVE-2025-24981 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-24981 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-24981? Upgrade @nuxtjs/mdc to 0.13.3 or later.