The link.href check in makeTagSafe (safe.ts, line 68-71) uses String.includes(), which is case-sensitive:
typescript if (key === 'href') { if (val.includes('javascript:') || val.includes('data:')) { return } next[key] = val }
Browsers treat URI schemes case-insensitively. DATA:text/css,... is the same as data:text/css,... to the browser, but 'DATA:...'.includes('data:') returns false.
PoC
javascript useHeadSafe({ link: [{ rel: 'stylesheet', href: 'DATA:text/css,body{display:none}' }] })
SSR output:
html <link rel="stylesheet" href="DATA:text/css,body{display:none}">
The browser loads this as a CSS stylesheet. An attacker can inject arbitrary CSS for UI redressing or data exfiltration via CSS attribute selectors with background-image callbacks.
Any case variation works: DATA:, Data:, dAtA:, JAVASCRIPT:, etc.
Suggested fix
typescript if (key === 'href') { const lower = val.toLowerCase() if (lower.includes('javascript:') || lower.includes('data:')) { return } next[key] = val }
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.domain) y': 'z' }] })
SSR output:
html <link data-x onload=alert(document.domain) y="z" rel="stylesheet" href="/valid-stylesheet.css">
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 <meta> 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 SAFEATTRRE = /^[a-zA-Z][a-zA-Z0-9\-]$/
function acceptDataAttrs(value: Record<string, string>) { return Object.fromEntries( Object.entries(value || {}).filter( ([key]) => (key === 'id' || key.startsWith('data-')) && SAFEATTRRE.test(key) ), ) }