DE0002: Code Access Security (CAS) shouldn't be used
When .NET Framework was originally created, it tried to provide a managed sandbox that allows restricting what parts of the process can do. Unfortunately, providing an additional security boundary on top of the operating system has been proven to be too difficult. As a result, there have been quite a few exploits that allows code to escape the managed sandbox.
We recommend not to use CAS for new code. For security purposes you should rely on operating system provided security boundaries, for example on Windows you can use Access Control Lists (ACLs).