Unhead is a document head and template manager. Prior to 2.1.13, useHeadSafe() is the composable that Nuxt's own documentation explicitly recommends for rendering user-supplied content in <head> safely. Internally, the hasDangerousProtocol() function in packages/unhead/src/plugins/safe.ts decodes HTML entities before checking for blocked URI schemes (javascript:, data:, vbscript:). The decoder uses two regular expressions with fixed-width digit caps. The HTML5 specification imposes no limit on leading zeros in numeric character references. When a padded entity exceeds the regex digit cap, the decoder silently skips it. The undecoded string is then passed to startsWith('javascript:'), which does not match. makeTagSafe() writes the raw value directly into SSR HTML output. The browser's HTML parser decodes the padded entity natively and constructs the blocked URI. This vulnerability is fixed in 2.1.13.

Affected: npm: unhead

##EVIDENCE

<img width="1900" height="855" alt="Screenshot_2026-03-25_090729" src="https://github.com/user-attachments/assets/3da93464-1caf-46ca-818f-46f8fe32ab50" /> <img width="1919" height="947" alt="Screenshot_2026-03-25_090715" src="https://github.com/user-attachments/assets/b27b1fc3-fa89-4864-99c9-4e6cff9a4e40" /> <img width="1918" height="925" alt="Screenshot_2026-03-25_090759" src="https://github.com/user-attachments/assets/9b8c94fa-d4f7-412e-ba14-214bc4103f4c" /> <img width="1912" height="812" alt="Screenshot_2026-03-25_090824" src="https://github.com/user-attachments/assets/3a4e1002-8811-453a-b08c-dfd1e42ebcf0" /> <img width="1846" height="409" alt="Screenshot_2026-03-22_090617" src="https://github.com/user-attachments/assets/9a595e13-ed18-464a-9d1a-0bb71dec96c9" />

| **Disclosed to Vercel H1** | 2026-03-22 (no response after 12 days) | | **Cross-reported here** | 2026-04-03 |

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## Summary

`useHeadSafe()` is the composable that Nuxt's own documentation explicitly recommends for rendering user-supplied content in `<head>` safely. Internally, the `hasDangerousProtocol()` function in `packages/unhead/src/plugins/safe.ts` decodes HTML entities before checking for blocked URI schemes (`javascript:`, `data:`, `vbscript:`). The decoder uses two regular expressions with fixed-width digit caps:

```js // Current — vulnerable const HtmlEntityHex = /&#x([0-9a-f]{1,6});?/gi const HtmlEntityDec = /&#(\d{1,7});?/g ```

The HTML5 specification imposes **no limit** on leadi…