diff --git a/backend/index.ts b/backend/index.ts
index 562765ed..b52e6525 100644
--- a/backend/index.ts
+++ b/backend/index.ts
@@ -15,6 +15,7 @@ import { Elysia } from 'elysia';
 import { corsMiddleware } from './middleware/cors';
 import { errorHandlerMiddleware } from './middleware/error-handler';
 import { loggerMiddleware } from './middleware/logger';
+import { securityMiddleware } from './middleware/security';
 
 // Database initialization
 import { initializeDatabase, closeDatabase } from './database';
@@ -87,6 +88,7 @@ const HOST = SERVER_ENV.HOST;
 const app = new Elysia()
 	// Apply middleware
 	.use(corsMiddleware)
+	.use(securityMiddleware)
 	.use(errorHandlerMiddleware)
 	.use(loggerMiddleware)
 
diff --git a/backend/middleware/security.ts b/backend/middleware/security.ts
new file mode 100644
index 00000000..8676cf25
--- /dev/null
+++ b/backend/middleware/security.ts
@@ -0,0 +1,61 @@
+import type { Elysia } from 'elysia';
+
+/**
+ * Security Headers Middleware
+ *
+ * Applies Content Security Policy (CSP) and other security headers to all responses.
+ *
+ * CSP Configuration Rationale:
+ * - default-src 'self': Restrict all resources to same-origin by default
+ * - script-src 'self' 'unsafe-inline' 'unsafe-eval': Monaco Editor requires eval for web workers and inline scripts
+ * - style-src 'self' 'unsafe-inline': Monaco and Tailwind use inline styles
+ * - img-src 'self' data: blob:: Support base64 images and blob URLs from browser preview
+ * - connect-src 'self' ws: wss:: Allow WebSocket connections for real-time chat and file operations
+ * - font-src 'self' data:: Support custom fonts and data URIs
+ * - frame-ancestors 'none': Prevent clickjacking by disallowing embedding in iframes
+ * - frame-src 'self': Allow iframes from same origin (browser preview)
+ * - worker-src 'self' blob:: Monaco Editor uses web workers from blob URLs
+ *
+ * Additional Security Headers:
+ * - X-Content-Type-Options: nosniff - Prevent MIME type sniffing
+ * - X-Frame-Options: DENY - Additional clickjacking protection (redundant with CSP frame-ancestors but defense-in-depth)
+ * - Referrer-Policy: strict-origin-when-cross-origin - Limit referrer information leakage
+ * - Permissions-Policy: Controls browser features (camera, microphone, geolocation, etc.)
+ */
+export function securityMiddleware(app: Elysia) {
+	return app.onAfterHandle(({ set }) => {
+		// Content Security Policy
+		set.headers['Content-Security-Policy'] = [
+			"default-src 'self'",
+			"script-src 'self' 'unsafe-inline' 'unsafe-eval'",
+			"style-src 'self' 'unsafe-inline'",
+			"img-src 'self' data: blob:",
+			"connect-src 'self' ws: wss:",
+			"font-src 'self' data:",
+			"frame-ancestors 'none'",
+			"frame-src 'self'",
+			"worker-src 'self' blob:"
+		].join('; ');
+
+		// Prevent MIME type sniffing
+		set.headers['X-Content-Type-Options'] = 'nosniff';
+
+		// Clickjacking protection (defense-in-depth with CSP frame-ancestors)
+		set.headers['X-Frame-Options'] = 'DENY';
+
+		// Referrer policy - limit information leakage
+		set.headers['Referrer-Policy'] = 'strict-origin-when-cross-origin';
+
+		// Permissions policy - restrict browser features
+		set.headers['Permissions-Policy'] = [
+			'camera=()',
+			'microphone=()',
+			'geolocation=()',
+			'payment=()',
+			'usb=()',
+			'magnetometer=()',
+			'gyroscope=()',
+			'accelerometer=()'
+		].join(', ');
+	});
+}