diff --git a/src/app.js b/src/app.js
index 826001a..f19f09f 100644
--- a/src/app.js
+++ b/src/app.js
@@ -3,6 +3,7 @@ import morgan from 'morgan';
 import helmet from 'helmet';
 import cors from 'cors';
 import cookieParser from 'cookie-parser';
+import xssClean from 'xss-clean';
 
 import { validateBody } from './middlewares/global/validateBody.js';
 import { notFound } from './middlewares/global/notFound.js';
@@ -58,29 +59,38 @@ app.use(
   })
 );
 
-// CORS configuration - allow local network access
+// CORS configuration
+const isProduction = process.env.NODE_ENV === 'production';
 const corsOptions = {
   origin: (origin, callback) => {
-    // Allow requests with no origin (mobile apps, curl, etc.)
-    if (!origin) return callback(null, true);
+    // Requests with no origin (curl, Postman, server-to-server)
+    // Only allow in development - in production this is a CORS bypass vector
+    if (!origin) {
+      if (isProduction) {
+        return callback(new Error('Not allowed by CORS'));
+      }
+      return callback(null, true);
+    }
 
-    // Allow localhost and local network IPs
-    const allowedPatterns = [
-      /^http:\/\/localhost(:\d+)?$/,
-      /^http:\/\/127\.0\.0\.1(:\d+)?$/,
-      /^http:\/\/192\.168\.\d{1,3}\.\d{1,3}(:\d+)?$/,
-      /^http:\/\/10\.\d{1,3}\.\d{1,3}\.\d{1,3}(:\d+)?$/,
-      /^http:\/\/172\.(1[6-9]|2\d|3[01])\.\d{1,3}\.\d{1,3}(:\d+)?$/,
-    ];
-
-    // Check if origin matches allowed patterns or CORS_ORIGIN env
+    // Check CORS_ORIGIN env variable first
     const corsOrigin = process.env.CORS_ORIGIN;
     if (corsOrigin && origin === corsOrigin) {
       return callback(null, true);
     }
 
-    if (allowedPatterns.some(pattern => pattern.test(origin))) {
-      return callback(null, true);
+    // In development, allow localhost and local network IPs
+    if (!isProduction) {
+      const allowedPatterns = [
+        /^https?:\/\/localhost(:\d+)?$/,
+        /^https?:\/\/127\.0\.0\.1(:\d+)?$/,
+        /^https?:\/\/192\.168\.\d{1,3}\.\d{1,3}(:\d+)?$/,
+        /^https?:\/\/10\.\d{1,3}\.\d{1,3}\.\d{1,3}(:\d+)?$/,
+        /^https?:\/\/172\.(1[6-9]|2\d|3[01])\.\d{1,3}\.\d{1,3}(:\d+)?$/,
+      ];
+
+      if (allowedPatterns.some(pattern => pattern.test(origin))) {
+        return callback(null, true);
+      }
     }
 
     callback(new Error('Not allowed by CORS'));
@@ -95,6 +105,9 @@ app.use(express.json({ limit: '10mb' }));
 app.use(express.urlencoded({ extended: true, limit: '10mb' }));
 app.use(cookieParser());
 
+// XSS sanitization - clean user input in body, params, and query
+app.use(xssClean());
+
 // Custom body validation middleware
 app.use(validateBody);
 
diff --git a/src/middlewares/global/errorHandler.js b/src/middlewares/global/errorHandler.js
index a86143b..017cf3b 100644
--- a/src/middlewares/global/errorHandler.js
+++ b/src/middlewares/global/errorHandler.js
@@ -6,7 +6,7 @@ export function errorHandler(err, req, res, next) {
     return next(err);
   }
 
-  // Log error
+  // Log full error server-side (including stack trace)
   logger.error('Neošetrená chyba', err);
 
   // Get status code
@@ -16,8 +16,8 @@ export function errorHandler(err, req, res, next) {
       ? res.statusCode
       : 500;
 
-  // Format error response
-  const errorResponse = formatErrorResponse(err, process.env.NODE_ENV === 'development');
+  // Never send stack traces to the client, even in development
+  const errorResponse = formatErrorResponse(err, false);
 
   res.status(statusCode).json(errorResponse);
 }
diff --git a/src/routes/ai-kurzy.routes.js b/src/routes/ai-kurzy.routes.js
index dfa1da3..2aa488c 100644
--- a/src/routes/ai-kurzy.routes.js
+++ b/src/routes/ai-kurzy.routes.js
@@ -16,19 +16,42 @@ if (!fs.existsSync(uploadsDir)) {
   fs.mkdirSync(uploadsDir, { recursive: true });
 }
 
+const ALLOWED_FILE_TYPES = [
+  'application/pdf',
+  'application/msword',
+  'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
+  'application/vnd.ms-excel',
+  'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+  'image/jpeg',
+  'image/png',
+  'image/gif',
+  'image/webp',
+  'text/plain',
+  'text/csv',
+];
+
 const storage = multer.diskStorage({
   destination: (req, file, cb) => {
     cb(null, uploadsDir);
   },
   filename: (req, file, cb) => {
+    // Sanitize filename to prevent path traversal
+    const sanitized = path.basename(file.originalname).replace(/[^a-zA-Z0-9._-]/g, '_');
     const uniqueSuffix = Date.now() + '-' + Math.round(Math.random() * 1E9);
-    cb(null, uniqueSuffix + '-' + file.originalname);
+    cb(null, uniqueSuffix + '-' + sanitized);
   }
 });
 
 const upload = multer({
   storage,
   limits: { fileSize: 10 * 1024 * 1024 }, // 10MB max
+  fileFilter: (req, file, cb) => {
+    if (ALLOWED_FILE_TYPES.includes(file.mimetype)) {
+      cb(null, true);
+    } else {
+      cb(new Error('Nepovolený typ súboru. Povolené: PDF, Word, Excel, obrázky, CSV, TXT.'));
+    }
+  },
 });
 
 // Validation schemas
diff --git a/src/routes/project.routes.js b/src/routes/project.routes.js
index dd118d8..0841563 100644
--- a/src/routes/project.routes.js
+++ b/src/routes/project.routes.js
@@ -10,10 +10,33 @@ import { createProjectSchema, updateProjectSchema } from '../validators/crm.vali
 import { z } from 'zod';
 
 // Configure multer for file uploads (memory storage)
+const ALLOWED_FILE_TYPES = [
+  'application/pdf',
+  'application/msword',
+  'application/vnd.openxmlformats-officedocument.wordprocessingml.document',
+  'application/vnd.ms-excel',
+  'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
+  'application/vnd.ms-powerpoint',
+  'application/vnd.openxmlformats-officedocument.presentationml.presentation',
+  'image/jpeg',
+  'image/png',
+  'image/gif',
+  'image/webp',
+  'text/plain',
+  'text/csv',
+];
+
 const upload = multer({
   storage: multer.memoryStorage(),
   limits: {
-    fileSize: 50 * 1024 * 1024, // 50MB max
+    fileSize: 20 * 1024 * 1024, // 20MB max
+  },
+  fileFilter: (req, file, cb) => {
+    if (ALLOWED_FILE_TYPES.includes(file.mimetype)) {
+      cb(null, true);
+    } else {
+      cb(new Error('Nepovolený typ súboru. Povolené: PDF, Word, Excel, PowerPoint, obrázky, CSV, TXT.'));
+    }
   },
 });