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middleware/security.js

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+const helmet = require('helmet');
+
+/**
+ * Helmet security configuration
+ * Helmet helps secure Express apps by setting various HTTP headers
+ */
+const helmetConfig = helmet({
+  // Content Security Policy
+  contentSecurityPolicy: {
+    directives: {
+      defaultSrc: ["'self'"],
+      styleSrc: ["'self'", "'unsafe-inline'"], // Allow inline styles for Swagger UI
+      scriptSrc: ["'self'", "'unsafe-inline'"], // Allow inline scripts for Swagger UI
+      imgSrc: ["'self'", "data:", "https:"],
+      connectSrc: ["'self'"],
+      fontSrc: ["'self'", "data:"],
+      objectSrc: ["'none'"],
+      mediaSrc: ["'self'"],
+      frameSrc: ["'none'"]
+    }
+  },
+  
+  // Cross-Origin-Embedder-Policy
+  crossOriginEmbedderPolicy: false, // Disabled for API compatibility
+  
+  // Cross-Origin-Opener-Policy
+  crossOriginOpenerPolicy: { policy: "same-origin" },
+  
+  // Cross-Origin-Resource-Policy
+  crossOriginResourcePolicy: { policy: "cross-origin" },
+  
+  // DNS Prefetch Control
+  dnsPrefetchControl: { allow: false },
+  
+  // Expect-CT (deprecated but included for older browsers)
+  expectCt: { maxAge: 86400 },
+  
+  // Frameguard (prevent clickjacking)
+  frameguard: { action: "deny" },
+  
+  // Hide Powered-By header
+  hidePoweredBy: true,
+  
+  // HTTP Strict Transport Security
+  hsts: {
+    maxAge: 31536000, // 1 year
+    includeSubDomains: true,
+    preload: true
+  },
+  
+  // IE No Open
+  ieNoOpen: true,
+  
+  // No Sniff (prevent MIME type sniffing)
+  noSniff: true,
+  
+  // Origin-Agent-Cluster
+  originAgentCluster: true,
+  
+  // Permitted Cross-Domain Policies
+  permittedCrossDomainPolicies: { permittedPolicies: "none" },
+  
+  // Referrer Policy
+  referrerPolicy: { policy: "no-referrer" }
+});
+
+/**
+ * Custom security headers middleware
+ * Only adds headers not already set by Helmet
+ */
+const customSecurityHeaders = (req, res, next) => {
+  // Add Permissions-Policy (not in Helmet)
+  res.setHeader('Permissions-Policy', 'geolocation=(), microphone=(), camera=()');
+  
+  // Prevent caching of sensitive data
+  if (req.path.includes('/api/auth') || req.path.includes('/api/admin') || req.path.includes('/api/users')) {
+    res.setHeader('Cache-Control', 'no-store, no-cache, must-revalidate, proxy-revalidate');
+    res.setHeader('Pragma', 'no-cache');
+    res.setHeader('Expires', '0');
+    res.setHeader('Surrogate-Control', 'no-store');
+  }
+  
+  next();
+};
+
+/**
+ * CORS configuration
+ */
+const getCorsOptions = () => {
+  const allowedOrigins = process.env.ALLOWED_ORIGINS 
+    ? process.env.ALLOWED_ORIGINS.split(',')
+    : ['http://localhost:3000', 'http://localhost:4200', 'http://localhost:5173'];
+
+  return {
+    origin: (origin, callback) => {
+      // Allow requests with no origin (mobile apps, Postman, etc.)
+      if (!origin) return callback(null, true);
+      
+      if (allowedOrigins.indexOf(origin) !== -1 || process.env.NODE_ENV === 'development') {
+        callback(null, true);
+      } else {
+        callback(new Error('Not allowed by CORS'));
+      }
+    },
+    credentials: true,
+    methods: ['GET', 'POST', 'PUT', 'DELETE', 'PATCH', 'OPTIONS'],
+    allowedHeaders: ['Content-Type', 'Authorization', 'x-guest-token'],
+    exposedHeaders: ['RateLimit-Limit', 'RateLimit-Remaining', 'RateLimit-Reset'],
+    maxAge: 86400 // 24 hours
+  };
+};
+
+/**
+ * Security middleware for API routes
+ */
+const secureApiRoutes = (req, res, next) => {
+  // Log security-sensitive operations
+  if (req.method !== 'GET' && req.path.includes('/api/admin')) {
+    const logger = require('../config/logger');
+    logger.logSecurityEvent(`Admin ${req.method} request`, req);
+  }
+  
+  next();
+};
+
+/**
+ * Prevent parameter pollution
+ * This middleware should be used after body parser
+ */
+const preventParameterPollution = (req, res, next) => {
+  // Whitelist of parameters that can have multiple values
+  const whitelist = ['category', 'difficulty', 'tags', 'keywords'];
+  
+  // Check for duplicate parameters
+  if (req.query) {
+    for (const param in req.query) {
+      if (Array.isArray(req.query[param]) && !whitelist.includes(param)) {
+        return res.status(400).json({
+          status: 'error',
+          message: `Parameter pollution detected: '${param}' should not have multiple values`
+        });
+      }
+    }
+  }
+  
+  next();
+};
+
+module.exports = {
+  helmetConfig,
+  customSecurityHeaders,
+  getCorsOptions,
+  secureApiRoutes,
+  preventParameterPollution
+};