screentinker/server/lib/oidc.js

'use strict';

/*
 * OpenID Connect — discovery, key handling and ID-token verification.
 *
 * This exists because the previous "OAuth" support verified nothing that mattered. The Google path
 * asked Google's tokeninfo endpoint whether an ACCESS token was valid and then trusted the email in
 * the reply; the Microsoft path handed a bearer token to Graph /me and trusted that. Neither ever
 * checked WHO THE TOKEN WAS ISSUED FOR, and an access token is not a proof of identity — it is a
 * bearer credential for some resource, minted for some application, and Graph will happily describe
 * the user behind a token issued to somebody else's app. Any site a user signs into that asks for
 * `email` or `User.Read` could replay that token here and be issued a session as that user.
 *
 * So identity now comes from an ID TOKEN and nothing else, and the token has to survive:
 *
 *   signature   against the provider's published JWKS, restricted to asymmetric algorithms
 *   iss         exactly the issuer discovery advertised
 *   aud         contains our client_id (and azp === client_id when the token carries one)
 *   exp/nbf     inside a small clock skew
 *   nonce       equal to the one WE generated for this login, which is what stops a token
 *               obtained elsewhere — even a correctly-audienced one — being replayed here
 *
 * Deliberately dependency-free beyond `jsonwebtoken`: Node can import a JWK straight into a
 * KeyObject, so there is no need for jwks-rsa and no second opinion about what a key is.
 */

const crypto = require('crypto');
const jwt = require('jsonwebtoken');

/*
 * `alg: "none"` is the oldest JWT attack there is, and HMAC is nearly as bad here: an HS256 token is
 * verified with a SHARED SECRET, and the only "key" we have for a provider is its PUBLIC one — which
 * an attacker also has, and could sign with. Only asymmetric families are ever acceptable.
 */
const ALLOWED_ALGS = ['RS256', 'RS384', 'RS512', 'ES256', 'ES384', 'ES512', 'PS256', 'PS384', 'PS512'];

// Providers rotate keys and publish new ones ahead of use, so a short cache is safe and a miss is
// cheap. Discovery changes far less often but is cached the same way for one reason: a provider
// outage should not be able to stall every login for as long as it lasts.
const DISCOVERY_TTL_MS = 60 * 60 * 1000;   // 1 hour
const JWKS_TTL_MS = 10 * 60 * 1000;        // 10 minutes
const FETCH_TIMEOUT_MS = 8000;

const discoveryCache = new Map();  // issuer -> { at, doc }
const jwksCache = new Map();       // jwks_uri -> { at, keys }

/** fetch with a timeout, because a hanging IdP must not hang a login forever. */
async function getJson(url) {
  const ctl = new AbortController();
  const timer = setTimeout(() => ctl.abort(), FETCH_TIMEOUT_MS);
  try {
    const res = await fetch(url, { signal: ctl.signal, redirect: 'follow' });
    if (!res.ok) throw new Error(`${url} responded ${res.status}`);
    return await res.json();
  } finally {
    clearTimeout(timer);
  }
}

/**
 * The provider's own description of itself.
 *
 * ⚠️ The discovered `issuer` is checked against the configured one. Discovery is fetched over TLS
 * from a URL derived from the issuer, so this is belt-and-braces — but a provider whose document
 * claims a DIFFERENT issuer is either misconfigured or hostile, and either way its tokens must not
 * be accepted under a name it does not own.
 */
async function discover(issuer) {
  const key = String(issuer).replace(/\/+$/, '');
  const hit = discoveryCache.get(key);
  if (hit && Date.now() - hit.at < DISCOVERY_TTL_MS) return hit.doc;

  const url = `${key}/.well-known/openid-configuration`;
  const doc = await getJson(url);

  const advertised = String(doc.issuer || '').replace(/\/+$/, '');
  if (advertised !== key) {
    throw new Error(`discovery issuer mismatch: configured ${key}, document says ${doc.issuer}`);
  }
  for (const required of ['authorization_endpoint', 'token_endpoint', 'jwks_uri']) {
    if (!doc[required]) throw new Error(`discovery for ${key} is missing ${required}`);
  }

  discoveryCache.set(key, { at: Date.now(), doc });
  return doc;
}

/**
 * The signing key for one token.
 *
 * An unknown `kid` forces ONE refresh: that is the normal shape of a key rotation, and refusing to
 * refetch would fail every login until the cache expired. It is bounded to one refresh per call so
 * a token quoting nonsense cannot be used to hammer the provider.
 */
async function keyForKid(jwksUri, kid) {
  let entry = jwksCache.get(jwksUri);
  const fresh = entry && Date.now() - entry.at < JWKS_TTL_MS;

  if (!fresh || !entry.keys.some((k) => k.kid === kid)) {
    const doc = await getJson(jwksUri);
    entry = { at: Date.now(), keys: Array.isArray(doc.keys) ? doc.keys : [] };
    jwksCache.set(jwksUri, entry);
  }

  const jwk = entry.keys.find((k) => k.kid === kid)
    // A provider with exactly one key may omit kid entirely; anything ambiguous is refused rather
    // than guessed, because "try each key until one verifies" is how you accept a key you did not mean to.
    || (!kid && entry.keys.length === 1 ? entry.keys[0] : null);
  if (!jwk) throw new Error(`no signing key for kid ${kid || '(none)'}`);

  return crypto.createPublicKey({ key: jwk, format: 'jwk' });
}

/**
 * Verify an ID token and return its claims.
 *
 * `nonce` is REQUIRED by this function even though the spec makes it conditional. Every flow here
 * is a browser login we initiated, so we always have one to compare — and it is the single check
 * that distinguishes "a token minted for us, now" from "a token minted for us at some point,
 * captured, and replayed".
 */
async function verifyIdToken(idToken, { issuer, clientId, nonce }) {
  if (!idToken || typeof idToken !== 'string') throw new Error('no id_token');
  if (!nonce) throw new Error('no nonce to verify against');

  const decoded = jwt.decode(idToken, { complete: true });
  if (!decoded || !decoded.header) throw new Error('id_token is not a JWT');
  if (!ALLOWED_ALGS.includes(decoded.header.alg)) {
    throw new Error(`refusing id_token algorithm ${decoded.header.alg}`);
  }

  const doc = await discover(issuer);
  const key = await keyForKid(doc.jwks_uri, decoded.header.kid);

  // jsonwebtoken checks signature, exp, nbf, iss and aud. The algorithm allowlist is passed
  // explicitly so the header cannot choose how it is verified.
  const claims = jwt.verify(idToken, key, {
    algorithms: ALLOWED_ALGS,
    issuer: doc.issuer,
    audience: clientId,
    clockTolerance: 60,
  });

  if (claims.nonce !== nonce) throw new Error('id_token nonce does not match this login');

  /*
   * azp names the party the token was issued TO when it differs from the audience. If it is present
   * it must be us: a token with our client_id merely in a multi-valued `aud`, issued to a different
   * application, is exactly the confused-deputy case this whole file exists to prevent.
   */
  if (claims.azp && claims.azp !== clientId) {
    throw new Error('id_token was issued to a different application');
  }
  if (!claims.sub) throw new Error('id_token has no subject');

  return claims;
}

/** PKCE S256. The verifier never leaves us; only its hash goes to the provider. */
function createPkce() {
  const verifier = crypto.randomBytes(32).toString('base64url');
  const challenge = crypto.createHash('sha256').update(verifier).digest('base64url');
  return { verifier, challenge, method: 'S256' };
}

const randomToken = () => crypto.randomBytes(32).toString('base64url');

/**
 * Exchange the authorization code.
 *
 * PKCE means a public client needs no secret, which is what lets a self-hoster configure a provider
 * without one. A secret is still sent when configured, because some providers (and some admins)
 * require confidential clients.
 */
async function exchangeCode({ issuer, clientId, clientSecret, code, redirectUri, verifier }) {
  const doc = await discover(issuer);
  const body = new URLSearchParams({
    grant_type: 'authorization_code',
    code,
    redirect_uri: redirectUri,
    client_id: clientId,
    code_verifier: verifier,
  });

  const headers = { 'Content-Type': 'application/x-www-form-urlencoded', Accept: 'application/json' };
  if (clientSecret) {
    // client_secret_basic is the form every provider accepts; client_secret_post is not universal.
    headers.Authorization = 'Basic ' + Buffer.from(`${encodeURIComponent(clientId)}:${encodeURIComponent(clientSecret)}`).toString('base64');
  }

  const ctl = new AbortController();
  const timer = setTimeout(() => ctl.abort(), FETCH_TIMEOUT_MS);
  let payload;
  try {
    const res = await fetch(doc.token_endpoint, { method: 'POST', headers, body, signal: ctl.signal });
    payload = await res.json().catch(() => ({}));
    if (!res.ok) {
      // The provider's own error is far more useful than "exchange failed" — a wrong redirect_uri
      // or an unregistered client is the overwhelmingly common setup mistake and it says so here.
      throw new Error(payload.error_description || payload.error || `token endpoint responded ${res.status}`);
    }
  } finally {
    clearTimeout(timer);
  }

  if (!payload.id_token) throw new Error('provider returned no id_token — is the openid scope requested?');
  return payload;
}

/** Test seam: drop cached discovery/JWKS so a test can change what a provider claims. */
function _resetCaches() {
  discoveryCache.clear();
  jwksCache.clear();
}

module.exports = {
  discover,
  verifyIdToken,
  exchangeCode,
  createPkce,
  randomToken,
  ALLOWED_ALGS,
  _resetCaches,
};