{"id":"CVE-2026-41067","aliases":["GHSA-j687-52p2-xcff"],"url":"https://o3.security/vulnerability/CVE-2026-41067","summary":"Astro: XSS via incomplete `` sanitization in `define:vars` allows case-insensitive and whitespace-based bypass","details":"## Summary\n\nThe `defineScriptVars` function in Astro's server-side rendering pipeline uses a case-sensitive regex `/<\\/script>/g` to sanitize values injected into inline ``, ``, or `` and inject arbitrary HTML/JavaScript.\n\n## Details\n\nThe vulnerable function is `defineScriptVars` at `packages/astro/src/runtime/server/render/util.ts:42-53`:\n\n```typescript\nexport function defineScriptVars(vars: Record) {\n\tlet output = '';\n\tfor (const [key, value] of Object.entries(vars)) {\n\t\toutput += `const ${toIdent(key)} = ${JSON.stringify(value)?.replace(\n\t\t\t/<\\/script>/g, // ← Case-sensitive, exact match only\n\t\t\t'\\\\x3C/script>',\n\t\t)};\\n`;\n\t}\n\treturn markHTMLString(output);\n}\n```\n\nThis function is called from `renderElement` at `util.ts:172-174` when a ``, ``, `` — HTML tag names are case-insensitive but the regex has no `i` flag.\n2. **Whitespace before `>`**: ``, ``, `` — after the tag name, the HTML tokenizer enters the \"before attribute name\" state on ASCII whitespace.\n3. **Self-closing slash**: `` — the tokenizer enters \"self-closing start tag\" state on `/`.\n\n`JSON.stringify()` does not escape `<`, `>`, or `/` characters, so all these payloads pass through serialization unchanged.\n\n**Execution flow:** User-controlled input (e.g., `Astro.url.searchParams`) → assigned to a variable → passed via `define:vars` on a `\n\n\n```\n\n**Step 2:** Ensure SSR is enabled in `astro.config.mjs`:\n\n```js\nexport default defineConfig({\n output: 'server'\n});\n```\n\n**Step 3:** Start the dev server and visit:\n\n```\nhttp://localhost:4321/?name=

\n```\n\n**Step 4:** View the HTML source. The output contains:\n\n```html\n

\";\n console.log(name);\n\n```\n\nThe browser's HTML parser matches `` case-insensitively, closing the script block. The `` is then parsed as HTML and the JavaScript in `onerror` executes.\n\n**Alternative bypass payloads:**\n\n```\n/?name=

\n/?name=

\n```\n\n## Impact\n\nAn attacker can execute arbitrary JavaScript in the context of a victim's browser session on any SSR Astro application that passes request-derived data to `define:vars` on a `` variants including case variations, whitespace, and self-closing forms.","published":"2026-04-24T16:57:22.940Z","modified":"2026-08-12T03:51:24.638988730Z","cvss":{"score":6.1,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"},"epss":{"score":0.00189,"percentile":0.08722,"asOf":"2026-08-10"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"astro","fixedVersion":"6.1.6"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/41xxx/CVE-2026-41067.json"},{"type":"ADVISORY","url":"https://github.com/withastro/astro/security/advisories/GHSA-j687-52p2-xcff"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-41067"},{"type":"PACKAGE","url":"https://github.com/withastro/astro"},{"type":"WEB","url":"https://github.com/withastro/astro/releases/tag/astro@6.1.6"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:24.638988730Z"}}