Unhead
CVE-2026-31860
MEDIUM
GHSA-g5xx-pwrp-g3fv
Severity by source
AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Primary rating from GitHub Advisory · only source for this CVE.
CVSS VectorGitHub Advisory
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Lifecycle Timeline
3Blast Radius
ecosystem impact- 6 npm packages depend on unhead (6 direct, 0 indirect)
Ecosystem-wide dependent count for version 2.1.11.
DescriptionGitHub Advisory
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.
AnalysisAI
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.
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:
tag.props = { ...acceptDataAttrs(prev), ...next }Then propsToString (propsToString.ts, line 26) interpolates property keys directly into the HTML string with no sanitization:
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:
useHeadSafe({
link: [{
rel: 'stylesheet',
href: '/valid-stylesheet.css',
'data-x onload=alert(document.domain) y': 'z'
}]
})SSR output:
<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):
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:
const SAFE_ATTR_RE = /^[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-')) && SAFE_ATTR_RE.test(key)
),
)
}Technical ContextAI
Cross-site scripting (XSS) allows injection of client-side scripts into web pages viewed by other users due to insufficient output encoding.
RemediationAI
A vendor patch is available — apply it immediately. Encode all user-supplied output contextually (HTML, JS, URL). Implement Content Security Policy (CSP) headers. Use HTTPOnly and Secure cookie flags.
Unhead's useHeadSafe() composable, explicitly recommended by Nuxt documentation for safely rendering user-supplied conte
The `link.href` check in `makeTagSafe` (safe.ts, line 68-71) uses `String.includes()`, which is case-sensitive: ```type
Same weakness CWE-79 – Cross-site Scripting (XSS)
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External POC / Exploit Code
Leaving vuln.today