### SummaryWhen 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 (``), it is possible to inject arbitrary HTML into the transformed output by supplying a malicious URL query string to `server.transformIndexHtml`.### ImpactOnly 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.### PatchesFixed in vite@5.0.5, vite@4.5.1, vite@4.4.12### DetailsSuppose `index.html` contains an inline module script:```html```This script is transformed into a proxy script like```html```due to Vite's HTML plugin:https://github.com/vitejs/vite/blob/7fd7c6cebfcad34ae7021ebee28f97b1f28ef3f3/packages/vite/src/node/plugins/html.ts#L429-L465When `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-L47so 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```It is possible to abuse this behavior by crafting a request URL to contain a malicious payload like```">```so a request to http://localhost:5173/?%22%3E%3C/script%3E%3Cscript%3Ealert(%27boom%27)%3C/script%3E produces HTML output like```html?html-proxy&index=0.js">```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 ImpactThis 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-L126However, 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```jsimport fs from 'node:fs/promises';import * as vite from 'vite';const 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).