Partial trust is a .NET security model in which application code executes with reduced permissions as a measure of protecting the local system from malicious use.
Partial trust is part of Microsoft's Code Access Security (CAS) model introduced with .NET 2.0.
Examples of partial trust: If you run a .NET application that resides on a network share, or you download an application via a web browser, the application will run with reduced permissions. The app will not be allowed direct access to the local file system or system registry, and will not be able to make certain kinds of network connections. The intent of partial trust is to reduce the risk of potentially malicious code downloaded from network locations from damaging or stealing information on the local machine.
With the release of .NET 4.0, portions of CAS have been deprecated or are no longer implemented, including the RequestOptional and LinkDemand SecurityActions, among others. http://msdn.microsoft.com/en-us/library/system.security.permissions.securityaction.aspx
