Make sure that using this pseudorandom number generator is safe here.

IoTSharp/Controllers/CaptchaController.cs

@@ -12,6 +12,7 @@ using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Security.Cryptography;
 
 namespace IoTSharp.Controllers
 {
@@ -21,7 +22,6 @@ namespace IoTSharp.Controllers
     [Route("api/[controller]/[action]")]
     public class CaptchaController : ControllerBase
     {
-        private readonly Random random = new Random();
         private readonly IEasyCachingProvider _caching;
         private readonly IWebHostEnvironment _hostingEnvironment;
   
@@ -84,7 +84,7 @@ namespace IoTSharp.Controllers
         private ModelCaptcha CreateImage()
         {
             using var buzzlefile = new MemoryStream(Properties.Resources.ResourceManager.GetObject("buzzle_template_png") as byte[]);
-            using var orginfile = new MemoryStream(Properties.Resources.ResourceManager.GetObject($"slide{random.Next(1, 9)}_jpg") as byte[]);
+            using var orginfile = new MemoryStream(Properties.Resources.ResourceManager.GetObject($"slide{RandomNumberGenerator.GetInt32(1,9)}_jpg") as byte[]);
             using var buzzlefilestream = new SKManagedStream(buzzlefile);
             using var orginfilestream = new SKManagedStream(orginfile);
             using var buzzle = SKBitmap.Decode(buzzlefilestream);