/***********************************************************************************************************************************************************************
 * File Name: originGuard.middleware.js
 * Type of Program: Middleware
 * Description: Three-layer server-side guard that blocks non-browser clients from reaching any API route.
 *
 *   ── Layer 1 — Fetch Metadata family  (ALL methods including GET) ─────────────────────────────────
 *
 *     1a) Sec-Fetch-Site presence + value
 *         Browsers (Chrome 76+, Firefox 90+, Safari 16.4+) automatically attach Sec-Fetch-Site on
 *         every request. It is a forbidden request header — JavaScript cannot set, override, or
 *         remove it. Its absence reliably signals a non-browser client. The value cross-site is
 *         also rejected; only same-origin, same-site, and none (direct navigation) are accepted.
 *
 *         Tools blocked (default configurations):
 *           ✓ Metasploit (Rex HTTP client)           — no Sec-Fetch-Site
 *           ✓ BurpSuite Repeater / Scanner           — no Sec-Fetch-Site
 *           ✓ Postman                                — no Sec-Fetch-Site
 *           ✓ curl / wget / httpie / python-requests — no Sec-Fetch-Site
 *           ✓ Nikto                                  — no Sec-Fetch-Site
 *           ✓ sqlmap                                 — no Sec-Fetch-Site
 *           ✓ dirb / gobuster / feroxbuster          — no Sec-Fetch-Site
 *           ✓ nmap HTTP scripts                      — no Sec-Fetch-Site
 *
 *     1b) Sec-Fetch-Mode + Sec-Fetch-Dest presence + valid combination
 *         Browsers that send Sec-Fetch-Site always send Mode and Dest too (Chrome 80+,
 *         Firefox 90+, Safari 16.4+). Missing headers or impossible combinations signal
 *         manual header injection. Only combinations expected on an API server are allowed:
 *           cors|empty          — standard fetch() call from a cross-origin SPA
 *           same-origin|empty   — same-origin fetch()
 *           navigate|document   — direct browser navigation to an API URL
 *
 *         Additional tools blocked:
 *           ✓ Scripts that fake only Sec-Fetch-Site  — missing Mode or Dest
 *           ✓ Scripts with wrong Mode+Dest combos    — no-cors, cors+document, etc.
 *
 *   ── Layer 2 — Browser presence signals  (ALL methods including GET) ─────────────────────────────
 *
 *     At least one browser-native header must be present:
 *       Sec-CH-UA       — Chromium client hint, forbidden in non-browser contexts
 *       Accept-Language — sent by all browsers (Chrome, Firefox, Safari)
 *     Absence of both is a strong automation signal that catches tools sophisticated enough to
 *     replicate the Sec-Fetch-* family but not the full browser header profile.
 *
 *   ── Layer 3 — Origin allowlist  (POST / PUT / PATCH / DELETE only) ──────────────────────────────
 *
 *     State-mutating requests must also carry an Origin header that matches ALLOWED_ORIGINS.
 *     Stops credential-stuffing and cross-origin mutation attempts from unlisted domains,
 *     even if an attacker replicated all browser headers above.
 *
 *   ── What this does NOT stop ──────────────────────────────────────────────────────────────────────
 *
 *     ✗ BurpSuite running as MITM proxy through a real browser session.
 *     ✗ Playwright / Puppeteer / Selenium controlling a real browser — they produce all correct
 *       Sec-Fetch-* headers, Sec-CH-UA, and Accept-Language automatically.
 *     ✗ A determined attacker who manually replicates all required headers.
 *       The only defences at that point are rate limiting and valid credentials.
 *
 *   ── Browser compatibility note ───────────────────────────────────────────────────────────────────
 *
 *     Sec-Fetch-Site is supported by Chrome 76+ (Aug 2019), Firefox 90+ (Jul 2021),
 *     and Safari 16.4+ (Mar 2023). Requests from browsers older than these thresholds
 *     will be rejected. Given this is a modern SPA (Vite + React), this is acceptable.
 *
 * Author: Kenneth Obsequio (@lash0000)
 * Date Created: Jun. 20, 2026
 * Date Modified: Jun. 29, 2026
 ***********************************************************************************************************************************************************************/
"use strict";

const R = require('../utils/response.util');

const ALLOWED = (process.env.ALLOWED_ORIGINS || process.env.APP_URL || '')
  .split(',')
  .map(o => o.trim())
  .filter(Boolean);

const MUTATION_METHODS = new Set(['POST', 'PUT', 'PATCH', 'DELETE']);

// Routes that legitimately receive a top-level cross-site navigation — a third-party
// IdP (Google) redirects the browser here after consent, so Sec-Fetch-Site is
// correctly "cross-site" even though the request is a real browser, not an attacker.
// Still gated on GET + navigate|document below, so this doesn't open the route to
// cross-site fetch()/XHR — only actual browser navigations.
const CROSS_SITE_NAVIGATION_PATHS = new Set(['/google/callback']);

// Valid Sec-Fetch-Mode + Sec-Fetch-Dest combinations expected on this API server.
const VALID_FETCH_COMBOS = new Set([
  'cors|empty',         // standard fetch() from cross-origin SPA
  'same-origin|empty',  // same-origin fetch()
  'navigate|document',  // direct browser navigation to an API URL
]);

module.exports = function originGuard(req, res, next) {
  // Dev bypass: set ORIGIN_GUARD_DISABLED=true in .env to allow Postman/curl through.
  // Hardcoded production lock — never bypassed even if the flag is accidentally set.
  if (process.env.ORIGIN_GUARD_DISABLED === 'true' && process.env.NODE_ENV !== 'production') {
    return next();
  }

  if (req.method === 'OPTIONS') return next(); // preflight — handled by cors()

  // ── Layer 1a: Sec-Fetch-Site must be present and not cross-site ───────────
  // Exception: OAuth callback routes receive a real cross-site top-level navigation
  // from the IdP's domain — allowed only in combination with the navigate|document
  // check in Layer 1b, so cross-site fetch()/XHR is still rejected everywhere.
  const fetchSite = req.headers['sec-fetch-site'];
  const isCrossSiteNavigationRoute = req.method === 'GET' && CROSS_SITE_NAVIGATION_PATHS.has(req.path);
  if (!fetchSite || (fetchSite === 'cross-site' && !isCrossSiteNavigationRoute)) {
    return R.error(res, 'Forbidden.', 403);
  }

  // ── Layer 1b: Fetch Metadata family must be complete and form a valid combo ─
  const fetchMode = req.headers['sec-fetch-mode'];
  const fetchDest = req.headers['sec-fetch-dest'];
  if (!fetchMode || !fetchDest || !VALID_FETCH_COMBOS.has(`${fetchMode}|${fetchDest}`)) {
    return R.error(res, 'Forbidden.', 403);
  }
  if (isCrossSiteNavigationRoute && `${fetchMode}|${fetchDest}` !== 'navigate|document') {
    return R.error(res, 'Forbidden.', 403);
  }

  // ── Layer 2: At least one browser-native fingerprint header must be present ─
  if (!req.headers['sec-ch-ua'] && !req.headers['accept-language']) {
    return R.error(res, 'Forbidden.', 403);
  }

  // ── Layer 3: Origin must be in allowlist for state-changing requests ───────
  if (MUTATION_METHODS.has(req.method)) {
    const origin = req.headers['origin'];
    if (!origin || !ALLOWED.includes(origin)) {
      return R.error(res, 'Forbidden.', 403);
    }
  }

  next();
};