The WebBrowser control is designed to work in full trust only. The HTML content displayed in the control can come from external Web servers and may contain unmanaged code in the form of scripts or Web controls. If you use the WebBrowser control in this situation, the control is no less secure than Internet Explorer would be, but the managed WebBrowser control does not prevent such unmanaged code from running.
For more information about security issues relating to the underlying ActiveX
WebBrowser control, see WebBrowser Control.