Enhance API functionality and security: Added rate limiting and domain validation across multiple API endpoints, improved error handling for missing or invalid parameters, and refactored email handling in contact form for better security and maintainability. Updated README.md with production build instructions and prerequisites.

backend/includes/functions.php

@@ -95,4 +95,56 @@ function generateCSRFToken() {
     }
     return $_SESSION['csrf_token'];
 }
+
+/**
+ * CSRF-Token prüfen und nach Erfolg invalidieren (Replay-Schutz)
+ */
+function validateCSRFToken($token) {
+    if (!isset($_SESSION['csrf_token']) || !is_string($token)) {
+        return false;
+    }
+    if (!hash_equals($_SESSION['csrf_token'], $token)) {
+        return false;
+    }
+    unset($_SESSION['csrf_token']);
+    return true;
+}
+
+/**
+ * Werte für E-Mail-Header bereinigen (Header-Injection verhindern)
+ */
+function sanitizeHeaderValue(string $value): string {
+    return str_replace(["\r", "\n", "\0"], '', trim($value));
+}
+
+/**
+ * Client-IP für Logging (Cloudflare / vertrauenswürdiger Reverse-Proxy)
+ */
+function getClientIP(): string {
+    if (!empty($_SERVER['HTTP_CF_CONNECTING_IP'])
+        && filter_var($_SERVER['HTTP_CF_CONNECTING_IP'], FILTER_VALIDATE_IP)) {
+        return $_SERVER['HTTP_CF_CONNECTING_IP'];
+    }
+
+    $remoteAddr = $_SERVER['REMOTE_ADDR'] ?? '0.0.0.0';
+    $isTrustedProxy = filter_var(
+        $remoteAddr,
+        FILTER_VALIDATE_IP,
+        FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE
+    ) === false;
+
+    if ($isTrustedProxy) {
+        foreach (['HTTP_X_REAL_IP', 'HTTP_X_FORWARDED_FOR'] as $header) {
+            if (empty($_SERVER[$header])) {
+                continue;
+            }
+            $ip = trim(explode(',', $_SERVER[$header])[0]);
+            if (filter_var($ip, FILTER_VALIDATE_IP)) {
+                return $ip;
+            }
+        }
+    }
+
+    return $remoteAddr;
+}
 ?>