How can the hidden webshell be accessed and controlled, and what defensive measures can detect it?

The hidden webshell is accessed via a virtual path that doesn't correspond to any physical file—for example, `https://target/owa/auth/fakepath31337/shell.aspx`. To avoid accidental detection, the webshell may require a custom HTTP header (e.g., `Value: 00HGAT3K0AXHV2RF2W0G`) before executing commands, redirecting unauthorized requests to a legitimate error page. Defenders can monitor for unexpected `ViewState` generators, abnormal temporary compilation files (`.compiled` files in `Temporary ASP.NET Files`), or unusual `VirtualPathProvider` registrations via runtime logging. Additionally, reviewing `web.config` for tampered `machineKey` values and restricting file write permissions are key preventive measures. For related techniques, see Implanting Backdoors into DLL Files Using BDF and Penetration Techniques - 'Fileless' Implementation Using Virtual Disks.