### Summary
When Vite's HTML transformation is invoked manually via `server.transformIndexHtml`, the original request URL is passed in unmodified, and the `html` being transformed contains inline module scripts (`<script type="module">...</script>`), it is possible to inject arbitrary HTML into the transformed output by supplying a malicious URL query string to `server.transformIndexHtml`.
### Impact
Only apps using `appType: 'custom'` and using the default Vite HTML middleware are affected. The HTML entry must also contain an inline script. The attack requires a user to click on a malicious URL while running the dev server. Restricted files aren't exposed to the attacker.
### Patches
Fixed in
[email protected],
[email protected],
[email protected]
### Details
Suppose `index.html` contains an inline module script:
```html
<script type="module">
// Inline script
</script>
```
This script is transformed into a proxy script like
```html
<script type="module" src="/index.html?html-proxy&index=0.js"></script>
```
due to Vite's HTML plugin:
https://github.com/vitejs/vite/blob/7fd7c6cebfcad34ae7021ebee28f97b1f28ef3f3/packages/vite/src/node/plugins/html.ts#L429-L465
When `appType: 'spa' | 'mpa'`, Vite serves HTML itself, and `htmlFallbackMiddleware` rewrites `req.url` to the canonical path of `index.html`,
https://github.com/vitejs/vite/blob/73ef074b80fa7252e0c46a37a2c94ba8cba46504/packages/vite/src/node/server/middlewares/htmlFallback.ts#L44-L47
so the `url` passed to `server.transformIndexHtml` is `/index.html`.
However, if `appType: 'custom'`, HTML is served manually, and if `server.transformIndexHtml` is called with the unmodified request URL (as the SSR docs suggest), then the path of the transformed `html-proxy` script varies with the request URL. For example, a request with path `/` produces
```html
<script type="module" src="/@id/__x00__/index.html?html-proxy&index=0.js"></script>
```
It is possible to abuse this behavior by crafting a request URL to contain a malicious payload like
```
"></script><script>alert('boom')</script>
```
so a request to http://localhost:5173/?%22%3E%3C/script%3E%3Cscript%3Ealert(%27boom%27)%3C/script%3E produces HTML output like
```html
<script type="module" src="/@id/__x00__/?"></script><script>alert("boom")</script>?html-proxy&index=0.js"></script>
```
which demonstrates XSS.
### PoC
- Example 1. Serving HTML from `vite dev` middleware with `appType: 'custom'`
- Go to https://stackblitz.com/edit/vitejs-vite-9xhma4?file=main.js&terminal=dev-html
- "Open in New Tab"
- Edit URL to set query string to `?%22%3E%3C/script%3E%3Cscript%3Ealert(%27boom%27)%3C/script%3E` and navigate
- Witness XSS:
- 
- Example 2. Serving HTML from SSR-style Express server (Vite dev server runs in middleware mode):
- Go to https://stackblitz.com/edit/vitejs-vite-9xhma4?file=main.js&terminal=server
- (Same steps as above)
- Example 3. Plain `vite dev` (this shows that vanilla `vite dev` is _not_ vulnerable, provided `htmlFallbackMiddleware` is used)
- Go to https://stackblitz.com/edit/vitejs-vite-9xhma4?file=main.js&terminal=dev
- (Same steps as above)
- You should _not_ see the alert box in this case
### Detailed Impact
This will probably predominantly affect [development-mode SSR](https://vitejs.dev/guide/ssr#setting-up-the-dev-server), where `vite.transformHtml` is called using the original `req.url`, per the docs:
https://github.com/vitejs/vite/blob/7fd7c6cebfcad34ae7021ebee28f97b1f28ef3f3/docs/guide/ssr.md?plain=1#L114-L126
However, since this vulnerability affects `server.transformIndexHtml`, the scope of impact may be higher to also include other ad-hoc calls to `server.transformIndexHtml` from outside of Vite's own codebase.
My best guess at bisecting which versions are vulnerable involves the following test script
```js
import fs from 'node:fs/promises';
import * as vite from 'vite';
const html = `
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
</head>
<body>
<script type="module">
// Inline script
</script>
</body>
</html>
`;
const server = await vite.createServer({ appType: 'custom' });
const transformed = await server.transformIndexHtml('/?%22%3E%3C/script%3E%3Cscript%3Ealert(%27boom%27)%3C/script%3E', html);
console.log(transformed);
await server.close();
```
and using it I was able to narrow down to #13581. If this is correct, then vulnerable Vite versions are 4.4.0-beta.2 and higher (which includes 4.4.0).