Astro: XSS in define:vars via incomplete tag sanitization ## Summary The `defineScriptVars` function in Astro's server-side rendering pipeline uses a case-sensitive regex `//g` to sanitize values injected into inline `` tags via the `define:vars` directive. HTML parsers close `` elements case-insensitively and also accept whitespace or `/` before the closing `>`, allowing an attacker to bypass the sanitization with payloads like ``, ``, or `` 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) { let output = ''; for (const [key, value] of Object.entries(vars)) { output += `const ${toIdent(key)} = ${JSON.stringify(value)?.replace( //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 `` element has `define:vars`: ```typescript if (name === 'script') { delete props.hoist; children = defineScriptVars(defineVars) + '\n' + children; } ``` The regex `//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**: ``, ``, `` — HTML tag names are case-insensitive but the regex has no `i` flag. 2. **Whitespace before `>`**: ``, ``, `` — after the tag name, the HTML tokenizer enters the "before attribute name" state on ASCII whitespace. 3. **Self-closing slash**: `` — 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 `` tag → `renderElement` → `defineScriptVars` → incomplete sanitization → injected into `` 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';

Hello

console.log(name); ``` **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= ``` **Step 4:** View the HTML source. The output contains: ```html const name = ""; console.log(name); ``` The browser's HTML parser matches `` case-insensitively, closing the script block. The `` is then parsed as HTML and the JavaScript in `onerror` executes. **Alternative bypass payloads:** ``` /?name= /?name= /?name= ``` ## 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 `` 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) { 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 `` variants including case variations, whitespace, and self-closing forms.