{"id":"CVE-2026-39315","revision":null,"records":[{"revision":"4336fb7377964fdcbf9a3cf473136ad9","observedAt":"2026-10-23T15:48:27.903+00:00","statement":{"source":"cve-program","id":"CVE-2026-39315","aliases":[],"title":"Unhead has a hasDangerousProtocol() bypass via leading-zero padded HTML entities in useHeadSafe()","description":"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 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.","state":0,"publishedAt":"2026-04-09T17:54:07.488+00:00","modifiedAt":"2026-04-13T15:38:52.507+00:00","affected":[{"ecosystem":"Vendor product","name":"unhead","ranges":[],"versions":[],"rawVersionEvidence":"{\n \"vendor\": \"unjs\",\n \"product\": \"unhead\",\n \"versions\": [\n {\n \"version\": \"< 2.1.13\",\n \"status\": \"affected\"\n }\n ]\n }"}],"metrics":[{"system":"CVSS 3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N","score":6.1,"severity":"MEDIUM"}],"references":["https://github.com/unjs/unhead/security/advisories/GHSA-95h2-gj7x-gx9w","https://github.com/unjs/unhead/commit/961ea781e091853812ffe17f8cda17105d2d2299","https://github.com/unjs/unhead/releases/tag/v2.1.13"],"sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-39315","license":"CVE Program Terms of Use","key":"cve-program:CVE-2026-39315","identifiers":["CVE-2026-39315"]},"retained":false,"historicalAffected":[]},{"revision":"2cc4b2d796374fb3b60c5c0d9530f7ef","observedAt":"2026-10-28T04:58:42.319+00:00","statement":{"source":"euvd","id":"EUVD-2026-20992","aliases":["CVE-2026-39315","GHSA-95h2-gj7x-gx9w"],"title":"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 safely. Internally, the hasDangerousProtocol(…","description":"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 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.","state":0,"publishedAt":null,"modifiedAt":null,"affected":[],"metrics":[{"system":"EUVD CVSS 3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N","score":6.1,"severity":null}],"references":["https://github.com/unjs/unhead/security/advisories/GHSA-95h2-gj7x-gx9w","https://github.com/unjs/unhead/commit/961ea781e091853812ffe17f8cda17105d2d2299","https://github.com/unjs/unhead/releases/tag/v2.1.13"],"sourceUrl":"https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-20992","license":"ENISA EUVD · source terms and attribution apply","enrichment":{"sourceStatus":null,"weaknesses":[],"productCriteria":[],"sourceDates":{"datePublished":"Apr 9, 2026, 5:54:07 PM","dateUpdated":"Apr 13, 2026, 3:38:52 PM"},"relatedAdvisories":[],"note":"EUVD source date text has no stated timezone and is retained without inventing a UTC instant. Related bulletin membership is not an alias, common cause or applicability claim. A missing vector with a zero score is treated as unrated. Complete vendor/product and underlying source evidence remains in the original JSON."},"key":"euvd:EUVD-2026-20992","identifiers":["EUVD-2026-20992","CVE-2026-39315","GHSA-95h2-gj7x-gx9w"]},"retained":false,"historicalAffected":[]},{"revision":"98e2794916664df1826bb40e39eb1d4c","observedAt":"2026-10-27T10:09:38.352+00:00","statement":{"source":"nvd","id":"CVE-2026-39315","aliases":[],"title":"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 safely. Internally, the hasDangerousProtocol(…","description":"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 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.","state":0,"publishedAt":"2026-04-09T18:17:01.433+00:00","modifiedAt":"2026-10-26T21:10:00.17+00:00","affected":[],"metrics":[{"system":"CVSS 3.1 · security-advisories@github.com · Secondary","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N","score":6.1,"severity":"MEDIUM"}],"references":["https://github.com/unjs/unhead/commit/961ea781e091853812ffe17f8cda17105d2d2299","https://github.com/unjs/unhead/releases/tag/v2.1.13","https://github.com/unjs/unhead/security/advisories/GHSA-95h2-gj7x-gx9w"],"sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-39315","license":"NIST NVD usage terms; underlying source attribution retained","enrichment":{"sourceStatus":"Analyzed","weaknesses":["CWE-184"],"productCriteria":["cpe:2.3:a:unjs:unhead:*:*:*:*:*:node.js:*:*"],"sourceDates":{"published":"2026-04-09T18:17:01.433","lastModified":"2026-10-26T21:10:00.170"},"relatedAdvisories":[],"note":"NVD analysis and CPE criteria are source context, not proof that a package or your environment is affected. Complete affected data, logical configurations and SSVC are retained in the original JSON."},"key":"nvd:CVE-2026-39315","identifiers":["CVE-2026-39315"]},"retained":false,"historicalAffected":[]},{"revision":"570d979fd0f1496b92e1f4d9b39364da","observedAt":"2026-10-24T13:17:17.793+00:00","statement":{"source":"osv","id":"GHSA-95h2-gj7x-gx9w","aliases":["CVE-2026-39315"],"title":"Unhead has a hasDangerousProtocol() bypass via leading-zero padded HTML entities in useHeadSafe()","description":"##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
\n
useHeadSafe bypass PoC
\n
View page source or run the curl command below.
\n
\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…
\n
\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. ``, `