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const express = require('express');
const cookieParser = require('cookie-parser');
const session = require('express-session');
const csrf = require('csurf'); // A common CSRF protection middleware

const app = express();

// Use cookie-parser middleware for parsing cookies
app.use(cookieParser());

// Use express-session middleware for session management
// IMPORTANT: Configure strong secret, secure: true, httpOnly: true for production
app.use(session({
  secret: 'a_very_secret_string_that_should_be_long_and_random',
  resave: false,
  saveUninitialized: true,
  cookie: { secure: true, httpOnly: true, sameSite: 'Lax' } // 'secure' for HTTPS only
}));

// Set up CSRF protection
// cookie: true tells csurf to use a cookie to store the token
const csrfProtection = csrf({ cookie: true });

// Middleware to expose CSRF token to templates
app.use((req, res, next) => {
  if (req.csrfToken) {
    res.locals.csrfToken = req.csrfToken();
  }
  next();
});

// Apply CSRF protection to routes that modify state (POST, PUT, DELETE)
app.get('/', csrfProtection, (req, res) => {
  // In a real application, you'd render an HTML form here
  // and inject res.locals.csrfToken into a hidden input field.
  res.send(`
    <h1>Home Page</h1>
    <p>CSRF Token: ${res.locals.csrfToken}</p>
    <form action="/process" method="POST">
      <input type="hidden" name="_csrf" value="${res.locals.csrfToken}">
      <input type="text" name="data" placeholder="Enter data">
      <button type="submit">Submit</button>
    </form>
  `);
});

app.post('/process', csrfProtection, (req, res) => {
  // Access req.body for form data if using body-parser
  res.send('Form data processed successfully! CSRF token validated.');
});

// Example without CSRF protection (DO NOT DO THIS FOR STATE-CHANGING ROUTES)
app.post('/insecure-process', (req, res) => {
  res.send('Insecure form data processed (NO CSRF protection!).');
});

// Error handling for CSRF issues
app.use((err, req, res, next) => {
  if (err.code === 'EBADCSRFTOKEN') {
    res.status(403).send('Invalid CSRF token.');
  } else {
    next(err);
  }
});

const PORT = 3000;
app.listen(PORT, () => {
  console.log(`Server running on http://localhost:${PORT}`);
});