GHSA-g5xx-pwrp-g3fv Unhead has XSS bypass in `useHeadSafe` via attribute name injection and case-sensitive protocol check

• npm unhead: < 2.1.11 → fixed in 2.1.11

Summary `useHeadSafe()` can be bypassed to inject arbitrary HTML attributes, including event handlers, into SSR-rendered `` 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) { 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. `` fires `onload` once the stylesheet loads, giving reliable script execution: ```javascript useHeadSafe({ link: [{ rel: 'stylesheet', href: '/valid-stylesheet.css', 'data-x onload=alert(document.domain) y': 'z' }] }) ``` SSR output: ```html ``` The browser parses `onload=alert(document.domain)` as its own attribute. Once the stylesheet loads, the handler fires. The same injection works on any tag type since `acceptDataAttrs` is applied to all of them at line 114. Here's the same thing on a `meta` tag (the injected attributes render, though `onclick` doesn't fire on non-interactive `` elements): ```javascript useHeadSafe({ meta: [{ name: 'description', content: 'legitimate content', 'data-x onclick=alert(document.domain) y': 'z' }] }) ``` Realistic scenario A Nuxt app accepts SEO metadata from a CMS or user profile. The developer uses `useHeadSafe()` as the docs recommend. An attacker puts a `data-*` key with spaces and an event handler into their input. The payload renders into the HTML on every page load. Suggested fix For vulnerability 1, validate that attribute names only contain characters legal in HTML attributes: ```typescript const SAFE_ATTR_RE = /^[a-zA-Z][a-zA-Z0-9\-]*$/ function acceptDataAttrs(value: Record) { return Object.fromEntries( Object.entries(value || {}).filter( ([key]) => (key === 'id' || key.startsWith('data-')) && SAFE_ATTR_RE.test(key) ), ) } ```