Get unstuck

Hint 1: A read followed by a write is not single-use

Another request can read unused state before your write.

Hint 2: Keep validation beside the effect

The service contract validates scope and expiry while claiming the token and applying the update.

Reveal the reference patch and explanation
Fixedrecovery.mjsJavaScript
export async function recoverAccount(request, service) {
  return await service.redeem(request.token, request.account) ? 'changed' : 'rejected';
}

The unsafe flow allows both concurrent callers to observe used=false before either writes. Atomic redeem marks the token used while validating account=alice, purpose=recovery and expires>100, then records one credential change and one session revocation. The sorted concurrent answers are changed/rejected; they do not identify which request won. Expiry exactly 100 fails. Passing bob instead of alice must reject with zero changes and zero revocations. The harness counts these effects; it does not handle or store a new real credential.

Compare it with your own fix, then rerun the tests. The editor is not changed when you open this explanation.

Inspect the actual test harness

The browser and the lab download run the same cases, including cases that catch common wrong fixes, such as comparing with one fixture value instead of the real rule. The download includes the complete test adapter and cryptographic fixtures.

Download cases.mjs
How the browser sandbox runs your code

Your code runs in QuickJS 2025-09-13 (quickjs-emscripten 0.32.0) inside a Web Worker, isolated from the page, with a 32 MiB memory limit and a 2.5-second time limit. The test cases and the pass or fail verdict run outside that sandbox, so changing built-ins such as Array.prototype.push cannot change the result. The lab download runs the same cases under Node.js, where your code runs directly.

The same as in current Node.js:

  • ES2025 syntax and built-ins, including Set methods such as intersection, iterator helpers, Promise.try, RegExp.escape and the RegExp v flag

Added to the sandbox so that it behaves like Node.js:

  • structuredClone, Array.fromAsync, queueMicrotask, atob and btoa
  • URL and URLSearchParams, parsed by your browser’s own URL implementation
  • TextEncoder and TextDecoder for UTF-8
  • Intl.Collator and a locale-aware String.prototype.localeCompare
  • console.log and related methods, whose output appears with your results

Not available in the browser sandbox:

  • Timers (setTimeout, setInterval), fetch and other network or storage APIs; the exercises are deterministic and offline
  • Node.js modules and globals: import, require, process and Buffer
  • Web Crypto (crypto.getRandomValues, crypto.subtle); each workshop supplies the cryptographic operations it needs
  • Intl other than Collator: toLocaleString and toLocaleDateString ignore the locale, so numbers have no digit grouping and dates use one fixed format in the device’s time zone
  • using declarations, Symbol.dispose and DisposableStack (explicit resource management)
  • Uint8Array.fromBase64 and toBase64, JSON.rawJSON, Atomics and Error.captureStackTrace

Use the lesson in your own system

This is a transaction-contract exercise, not a complete recovery service. Production needs high-entropy credentials, hashed storage, bounded attempts, delivery-channel protection, secure new-credential handling, notifications, auditing and durable rollback semantics.

Your next task: What happens if your recovery process commits the credential change but crashes before invalidating the old session?

OWASP forgot-password guidance

Related guide: Account recovery: password resets, help-desk resets and account linking