State Parameter

1. Attacker initiates OAuth flow for victim's account
2. Attacker gets authorization code in callback
3. Attacker tricks victim into visiting callback URL with attacker's code
4. Victim's app exchanges code and links attacker's Zoom account to victim's app account
5. Attacker now has access to victim's app data

1. App generates random state before redirecting to OAuth
2. App stores state in user's session
3. Zoom includes state in callback
4. App verifies state matches session
5. If state doesn't match → Reject (CSRF detected)

const crypto = require('crypto');

app.get('/auth', (req, res) => {
  // Generate cryptographically secure random state
  const state = crypto.randomBytes(16).toString('hex');
  
  // Store in session (server-side)
  req.session.oauthState = state;
  
  const authURL = new URL('https://zoom.us/oauth/authorize');
  authURL.searchParams.set('response_type', 'code');
  authURL.searchParams.set('client_id', process.env.ZOOM_CLIENT_ID);
  authURL.searchParams.set('redirect_uri', process.env.ZOOM_REDIRECT_URL);
  authURL.searchParams.set('state', state); // Include state
  
  res.redirect(authURL.toString());
});

app.get('/callback', async (req, res) => {
  const { code, state } = req.query;
  const sessionState = req.session.oauthState;
  
  // Verify state matches
  if (state !== sessionState) {
    return res.status(403).send('Invalid state parameter - possible CSRF attack');
  }
  
  // Clean up state (one-time use)
  delete req.session.oauthState;
  
  // Proceed with token exchange
  const tokens = await exchangeCodeForToken(code);
  // ...
});

┌─────────────┐               ┌──────────────┐               ┌──────────────┐
│  Your App   │               │  User Session│               │  Zoom OAuth  │
└──────┬──────┘               └──────┬───────┘               └──────┬───────┘
       │                             │                              │
       │ 1. Generate state           │                              │
       │    state = "abc123"         │                              │
       │                             │                              │
       │ 2. Store in session         │                              │
       │────────────────────────────>│ session.oauthState = "abc123"
       │                             │                              │
       │ 3. Redirect to authorize with state                        │
       │https://zoom.us/oauth/authorize?state=abc123                │
       │───────────────────────────────────────────────────────────>│
       │                             │                              │
       │ 4. User authorizes          │                              │
       │                             │                              │
       │ 5. Redirect to callback with state                         │
       │  /callback?code=xyz&state=abc123                           │
       │<───────────────────────────────────────────────────────────│
       │                             │                              │
       │ 6. Retrieve session state   │                              │
       │<────────────────────────────│ sessionState = "abc123"      │
       │                             │                              │
       │ 7. Verify state === sessionState                           │
       │    "abc123" === "abc123" ✓  │                              │
       │                             │                              │
       │ 8. Exchange code for token  │                              │
       │───────────────────────────────────────────────────────────>│
       │                             │                              │

// ❌ WRONG: Accepting any state
app.get('/callback', async (req, res) => {
  const { code } = req.query;
  // No state verification!
  await exchangeCodeForToken(code);
});

// ✅ CORRECT: Verifying state
app.get('/callback', async (req, res) => {
  const { code, state } = req.query;
  
  if (state !== req.session.oauthState) {
    return res.status(403).send('CSRF detected');
  }
  
  await exchangeCodeForToken(code);
});

// ❌ WRONG: Predictable state
const state = Date.now().toString(); // Attacker can predict!

// ✅ CORRECT: Cryptographically random state
const state = crypto.randomBytes(16).toString('hex');

// ❌ WRONG: Not deleting state after use
if (state === req.session.oauthState) {
  // State remains in session - can be reused!
}

// ✅ CORRECT: Delete state after verification
if (state === req.session.oauthState) {
  delete req.session.oauthState; // One-time use
}

app.get('/auth', (req, res) => {
  // Generate state (CSRF protection)
  const state = crypto.randomBytes(16).toString('hex');
  req.session.oauthState = state;
  
  // Generate PKCE (authorization code interception protection)
  const { code_verifier, code_challenge } = generatePKCE();
  req.session.pkceVerifier = code_verifier;
  
  const authURL = new URL('https://zoom.us/oauth/authorize');
  authURL.searchParams.set('response_type', 'code');
  authURL.searchParams.set('client_id', CLIENT_ID);
  authURL.searchParams.set('redirect_uri', REDIRECT_URI);
  authURL.searchParams.set('state', state); // CSRF protection
  authURL.searchParams.set('code_challenge', code_challenge); // PKCE
  authURL.searchParams.set('code_challenge_method', 'S256');
  
  res.redirect(authURL.toString());
});

app.get('/callback', async (req, res) => {
  const { code, state } = req.query;
  
  // Verify state (CSRF)
  if (state !== req.session.oauthState) {
    return res.status(403).send('CSRF detected');
  }
  
  // Exchange with code_verifier (PKCE)
  const code_verifier = req.session.pkceVerifier;
  const tokens = await exchangeCode(code, code_verifier);
  
  // Clean up
  delete req.session.oauthState;
  delete req.session.pkceVerifier;
});

// Store state in secure storage
const state = generateRandomString();
await SecureStore.setItemAsync('oauth_state', state);

// Verify in callback
const storedState = await SecureStore.getItemAsync('oauth_state');
if (receivedState !== storedState) {
  throw new Error('CSRF detected');
}

// Clean up
await SecureStore.deleteItemAsync('oauth_state');