What are virtual files in ASP.NET and how can they be used to hide a webshell?

Virtual files in ASP.NET are created using the `VirtualPathProvider` class, which allows a file to be dynamically compiled and accessed even though it doesn't physically exist on the server's file system. By overriding the `FileExists` and `GetFile` methods, an attacker can serve malicious ASP.NET code on demand, effectively hiding the webshell's content. However, this method still depends on a physical file (e.g., `test1.aspx`) to trigger the virtual path, making it less stealthy on its own. For a deeper dive into the technique, see the original article on Penetration Techniques - Hiding ASP.NET Webshells Using Virtual Files.