Vite XSS vulnerability in `server.transformIndexHtml` via URL payload

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 vite@5.0.5, vite@4.5.1, vite@4.4.12

Details

Suppose index.html contains an inline module script:

<script type="module">
  // Inline script
</script>

This script is transformed into a proxy script like

<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

<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

<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:
  • image
• 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, 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

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).