Astro: XSS in define:vars via incomplete </script> tag sanitization
Date: 2026-04
Affected: npm/astro
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-41067
CWE: CWE-79
CVSS: 6.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N)

## Summary The `defineScriptVars` function in Astro's server-side rendering pipeline uses a case-sensitive regex `/<\/script>/g` to sanitize values injected into inline `<script>` tags via the `define:vars` directive. HTML parsers close `<script>` elements case-insensitively and also accept whitespace or `/` before the closing `>`, allowing an attacker to bypass the sanitization with payloads like `</Script>`, `</script >`, or `</script/>` and inject arbitrary HTML/JavaScript. ## Details The vulnerable function is `defineScriptVars` at `packages/astro/src/runtime/server/render/util.ts:42-53`: ```typescript export function defineScriptVars(vars: Record<any, any>) { let output = ''; for (const [key, value] of Object.entries(vars)) { output += `const ${toIdent(key)} = ${JSON.stringify(value)?.replace( /<\/script>/g, // ← Case-sensitive, exact match only '\\x3C/script>', )};\n`; } return markHTMLString(output); } ``` This function is called from `renderElement` at `util.ts:172-174` when a `<script>` element has `define:vars`: ```typescript if (name === 'script') { delete props.hoist; children = defineScriptVars(defineVars) + '\n' + children; } ``` The regex `/<\/script>/g` fails to match three classes of closing script tags that HTML parsers accept per the [HTML specification §13.2.6.4](https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inbody): 1. **Case variations**: `</Script>`, `</SCRIPT>`, `</sCrIpT>` — HTML tag names are case-inse


Mitigations:

Architectural layers (MAESTRO):