Improve path validation for Custom Script notifications

src/NzbDrone.Core/Notifications/CustomScript/CustomScript.cs

@@ -222,6 +222,16 @@ namespace NzbDrone.Core.Notifications.CustomScript
                 failures.Add(new NzbDroneValidationFailure("Path", "File does not exist"));
             }
 
+            foreach (var systemFolder in SystemFolders.GetSystemFolders())
+            {
+                if (systemFolder.IsParentPath(Settings.Path))
+                {
+                    failures.Add(new NzbDroneValidationFailure("Path", $"Must not be a descendant of '{systemFolder}'"));
+                }
+            }
+
+            if (failures.Empty())
+            {
                 try
                 {
                     var environmentVariables = new StringDictionary();
@@ -239,6 +249,7 @@ namespace NzbDrone.Core.Notifications.CustomScript
                     _logger.Error(ex);
                     failures.Add(new NzbDroneValidationFailure(string.Empty, ex.Message));
                 }
+            }
 
             return new ValidationResult(failures);
         }