Get unstuck

Hint 1: Inspect before joining

A path resolver can normalize away the evidence of a forbidden parent segment. This policy rejects it before joining.

Hint 2: Include archive metadata

Reject symlink entries even when their entry name is ordinary. A link target can point elsewhere.

Reveal the reference patch and explanation
Fixedarchive-policy.mjsJavaScript
export function allowEntry(entry) {
  if (!entry || entry.kind !== 'file' || typeof entry.name !== 'string') return false;
  const name = entry.name;
  if (!name || name.length > 200 || /[\\:\x00-\x1f\x7f]/.test(name)) return false;
  return name.split('/').every(part => part !== '' && part !== '.' && part !== '..');
}

docs/guide.txt passes because both segments are ordinary relative names. ../outside.txt and docs/../../outside.txt fail when any segment equals ..; /etc/settings and docs//guide.txt fail because splitting produces an empty segment. A symlink fails even if its name looks safe. This function returns a decision only: it neither resolves a filesystem path nor proves protection against existing symlinks, write races, duplicates or decompression limits.

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

Name filtering is one boundary. A real extractor must also avoid existing symlinks and races, verify the final filesystem destination, bound count and expanded bytes, and extract in an isolated directory. These tests execute the policy; they do not create files or validate an archive library.

Your next task: How does your extraction library identify symlinks, hard links, duplicate entries and decompression limits?

OWASP file upload guidance

Related guide: Extract archives without escaping the destination