Unhead is a document head and template manager. Prior to 2.1.11, useHeadSafe() can be bypassed to inject arbitrary HTML attributes, including event handlers, into SSR-rendered <head> tags. This is the composable that Nuxt docs recommend for safely handling user-generated content. The acceptDataAttrs function (safe.ts, line 16-20) allows any property key starting with data- through to the final HTML. It only checks the prefix, not whether the key contains spaces or other characters that break HTML attribute parsing. This vulnerability is fixed in 2.1.11.

Affected: npm: unhead

## Summary

`useHeadSafe()` can be bypassed to inject arbitrary HTML attributes, including event handlers, into SSR-rendered `<head>` tags. This is the composable that Nuxt docs recommend for safely handling user-generated content.

## Details

**XSS via `data-*` attribute name injection**

The `acceptDataAttrs` function (safe.ts, line 16-20) allows any property key starting with `data-` through to the final HTML. It only checks the prefix, not whether the key contains spaces or other characters that break HTML attribute parsing.

```typescript function acceptDataAttrs(value: Record<string, string>) { return Object.fromEntries( Object.entries(value || {}).filter(([key]) => key === 'id' || key.startsWith('data-')), ) } ```

This result gets merged into every tag's props at line 114:

```typescript tag.props = { ...acceptDataAttrs(prev), ...next } ```

Then `propsToString` (propsToString.ts, line 26) interpolates property keys directly into the HTML string with no sanitization:

```typescript attrs += value === true ? ` ${key}` : ` ${key}="${encodeAttribute(value)}"` ```

A space in the key breaks out of the attribute name. Everything after the space becomes separate HTML attributes.

### PoC

The most practical vector uses a `link` tag. `<link rel="stylesheet">` fires `onload` once the stylesheet loads, giving reliable script execution:

```javascript useHeadSafe({ link: [{ rel: 'stylesheet', href: '/valid-stylesheet.css', 'data-x onload=alert(document.d…