CVE-2026-31860 — unhead
Fix: unjs/unhead@9ecc4f9CVE-2026-31860 is a Cross-site Scripting (XSS) vulnerability in unhead. A fix is available for unhead — see the affected versions and patch details below.
Unhead has a XSS bypass in `useHeadSafe` via attribute name injection and case-sensitive protocol check
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-31860.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
unheadnpmDescription
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)
),
)
}
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | unhead | all versions | 2.1.11npm install unhead@2.1.11 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for unhead, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update unhead to 2.1.11 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-31860 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-31860 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-31860. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-31860 in your dependencies?
O3 Security finds CVE-2026-31860 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.