Void-Homelab/lib/auth/cf_access.js

// Trust a verified Cloudflare Access identity as the Void owner, so browser
// requests that came through the CF tunnel don't need the owner token copied
// onto every device. SECURITY: we verify the Access JWT (`Cf-Access-Jwt-Assertion`)
// cryptographically — signature against the team's JWKS, audience = our app's AUD,
// and email in the owner allow-list. The plain `Cf-Access-Authenticated-User-Email`
// header is NEVER trusted on its own (it would be spoofable on the LAN-direct path).
// Everything fails CLOSED: any miss/error returns null → falls back to token auth.
//
// Opt-in via env (absent → feature disabled, behaviour unchanged):
//   CF_ACCESS_TEAM_DOMAIN=hynesy.cloudflareaccess.com
//   CF_ACCESS_AUD=<application audience tag>
//   CF_ACCESS_OWNER_EMAILS=mrhynesy@gmail.com[,other@…]
import crypto from 'node:crypto';

const CERTS_TTL_MS = 60 * 60 * 1000;
let jwks = { at: 0, keys: {} };          // kid -> KeyObject
export function _resetJwksCache() { jwks = { at: 0, keys: {} }; }

function b64urlBuf(s) { return Buffer.from(s.replace(/-/g, '+').replace(/_/g, '/'), 'base64'); }
function b64urlJson(s) { return JSON.parse(b64urlBuf(s).toString('utf8')); }

async function getKeys(teamDomain, fetchImpl) {
  if (Date.now() - jwks.at < CERTS_TTL_MS && Object.keys(jwks.keys).length) return jwks.keys;
  const res = await fetchImpl(`https://${teamDomain}/cdn-cgi/access/certs`, { signal: AbortSignal.timeout(5000) });
  if (!res.ok) throw new Error(`jwks fetch ${res.status}`);
  const { keys } = await res.json();
  const map = {};
  for (const jwk of keys || []) map[jwk.kid] = crypto.createPublicKey({ key: jwk, format: 'jwk' });
  jwks = { at: Date.now(), keys: map };
  return map;
}

/**
 * Verify a Cloudflare Access JWT. Returns the claims if valid; throws otherwise.
 */
export async function verifyAccessJwt(jwt, { teamDomain, aud, fetchImpl = fetch, now = Date.now } = {}) {
  const [h, p, s] = String(jwt).split('.');
  if (!h || !p || !s) throw new Error('malformed jwt');
  const header = b64urlJson(h);
  if (header.alg !== 'RS256') throw new Error(`unexpected alg ${header.alg}`);

  let keys = await getKeys(teamDomain, fetchImpl);
  let key = keys[header.kid];
  if (!key) { jwks.at = 0; keys = await getKeys(teamDomain, fetchImpl); key = keys[header.kid]; } // key rotation
  if (!key) throw new Error('unknown kid');

  const ok = crypto.verify('RSA-SHA256', Buffer.from(`${h}.${p}`), key, b64urlBuf(s));
  if (!ok) throw new Error('bad signature');

  const claims = b64urlJson(p);
  const auds = Array.isArray(claims.aud) ? claims.aud : [claims.aud];
  // `aud` may be a single value or a comma-separated list (multiple CF Access
  // apps front the same origin — e.g. void.hynesy.com and void2-app.hynesy.com
  // during the 8b cutover). Accept the token if it carries ANY allowed aud.
  const wanted = (Array.isArray(aud) ? aud : String(aud).split(','))
    .map((a) => a.trim()).filter(Boolean);
  if (!auds.some((a) => wanted.includes(a))) throw new Error('aud mismatch');
  const t = Math.floor(now() / 1000);
  if (claims.exp && t > claims.exp) throw new Error('expired');
  if (claims.nbf && t < claims.nbf - 60) throw new Error('not yet valid');
  return claims;
}

export function config(env = process.env) {
  const { CF_ACCESS_TEAM_DOMAIN, CF_ACCESS_AUD, CF_ACCESS_OWNER_EMAILS } = env;
  if (!CF_ACCESS_TEAM_DOMAIN || !CF_ACCESS_AUD || !CF_ACCESS_OWNER_EMAILS) return null;
  return {
    teamDomain: CF_ACCESS_TEAM_DOMAIN,
    aud: CF_ACCESS_AUD,
    ownerEmails: CF_ACCESS_OWNER_EMAILS.split(',').map(e => e.trim().toLowerCase()).filter(Boolean)
  };
}

/**
 * If the request carries a valid Access JWT for an allow-listed owner email,
 * return that email; otherwise null. Never throws (fails closed).
 */
export async function accessOwnerEmail(req, cfg = config(), opts = {}) {
  if (!cfg) return null;
  const jwt = req.headers['cf-access-jwt-assertion'];
  if (!jwt) return null;
  try {
    const claims = await verifyAccessJwt(jwt, { teamDomain: cfg.teamDomain, aud: cfg.aud, ...opts });
    const email = String(claims.email || claims.identity || '').toLowerCase();
    return cfg.ownerEmails.includes(email) ? email : null;
  } catch {
    return null;
  }
}