Vitest: Path Traversal / Arbitrary File Read via @vitest/mocker Redirect Mock

Summary

@vitest/mocker registers a redirect mock's target path without validating it
against the dev server's file-serving allowlist. An attacker who can reach the
dev server's WebSocket can register a redirect mock pointing outside the project
root; when the mocked module is requested, the plugin's load hook returns
readFile(<attacker path>) as the module source, disclosing local files.

This is exploitable without authentication only through the public
mockerPlugin / standalone interceptorPlugin exports (used by third-party dev
servers), which register the handler on Vite's unauthenticated HMR socket.
Vitest's own browser mode registers mocks over a token-authenticated RPC and
is not remotely reachable by default (see Scope).

Affected code

packages/mocker/src/node/interceptorPlugin.ts.

The load hook is the file-read sink:

if (mock.type === 'redirect') {
  return readFile(mock.redirect, 'utf-8')
}

mock.redirect is derived from client input at registration time with no
boundary check:

if (event.type === 'redirect') {
  const redirectUrl = new URL(event.redirect)
  event.redirect = join(server.config.root, redirectUrl.pathname)
}
registry.register(event)

There is no server.fs.allow / server.fs.deny check and no assertion that the
resolved path stays within the project root.

Registration paths and trust boundaries

• Public mockerPlugin / interceptorPlugin (unauthenticated). In
  configureServer, the plugin registers server.ws.on('vitest:interceptor:register', …)
  on Vite's HMR WebSocket. That socket performs no token, Origin, or same-origin
  check, so any client that can reach it can register a redirect mock. This is
  the path the "unauthenticated" impact applies to.
• Vitest browser mode (authenticated). Mocks register over the browser RPC
  (registerMock), which sits behind a per-run token (isValidApiRequest, a
  random api.token). The interceptor's configureServer socket is not used for
  registration here (in v5 it does not run at all, as the plugin is injected per
  environment). The same missing boundary check exists on the authenticated RPC
  path, but reaching it requires the token, so it is not a remote-unauthenticated
  read.

Path handling

new URL(redirect).pathname combined with join(root, pathname) does not
confine reads to the root:

• Special/hierarchical schemes (file:, http:) are normalized by WHATWG URL,
  so .. segments are collapsed and the result stays under the root. Payloads of
  the form file:///../../etc/passwd do not escape.
• A non-special (opaque) scheme preserves .. in pathname, so
  join(root, "../../…/etc/passwd") resolves outside the root and reads an
  arbitrary file.

Even without escaping the root, the missing server.fs check allows reading any
in-root file the dev server would otherwise refuse to serve (for example an
in-root .env or source that is denied by server.fs.deny).

Scope / preconditions

• This is a development-server issue. The dev server binds to localhost by
  default and is not reachable from the network unless the developer exposes it
  (server.host / 0.0.0.0, a LAN bind, or a proxy).
• A raw (non-browser) client against a reachable server bypasses browser origin
  and CORS protections entirely and can both register the mock and read the
  response.
• A browser-based drive-by against a localhost server is substantially mitigated
  by Vite defaults: the default CORS origin allowlist is limited to localhost
  origins, and server.allowedHosts blocks DNS-rebinding, so a cross-origin page
  cannot read the file contents back.

Impact

Disclosure of local files readable by the dev-server process (source, in-root
.env/secrets, and, via the opaque-scheme payload, files outside the project
root). No integrity or availability impact.

Affected versions

Present since @vitest/mocker was introduced.

• Affected: @vitest/mocker >= 2.1.0 (shipped in vitest and
  @vitest/browser >= 2.1.0), through 4.1.x and the 5.0.0 pre-releases.
• Fixed: Vitest 4.1.11 and 5.0.0. Older majors (2.1.x, 3.x) are not
  maintained and are not planned to receive the fix.

Fix

Validate the resolved redirect target against Vite's file-serving allowlist
(isFileLoadingAllowed) before registering it, at every registration site, and
stop registering the interceptor WebSocket events in Vitest's browser mode
(mocks there flow through the authenticated RPC).