Code Access Security and Partially Trusted Code
The .NET Framework provides a mechanism for the enforcement of varying levels of trust on different code running in the same application called Code Access Security (CAS). Code Access Security in .NET Framework should not be used as a mechanism for enforcing security boundaries based on code origination or other identity aspects. We are updating our guidance to reflect that Code Access Security and Security-Transparent Code will not be supported as a security boundary with partially trusted code, especially code of unknown origin. We advise against loading and executing code of unknown origins without putting alternative security measures in place.
This policy applies to all versions of .NET Framework, but does not apply to the .NET Framework included in Silverlight.
A link demand causes a security check during just-in-time compilation and checks only the immediate calling assembly of your code. Linking occurs when your code is bound to a type reference, including function pointer references and method calls. If the calling assembly does not have sufficient permission to link to your code, the link is not allowed and a runtime exception is thrown when the code is loaded and run. Link demands can be overridden in classes that inherit from your code.
Note that a full stack walk is not performed with this type of demand and that your code is still susceptible to luring attacks. For example, if a method in assembly A is protected by a link demand, a direct caller in assembly B is evaluated based on the permissions of Assembly B. However, the link demand will not evaluate a method in assembly C if it indirectly calls the method in assembly A using the method in assembly B. The link demand specifies only the permissions direct callers in the immediate calling assembly must have to link to your code. It does not specify the permissions all callers must have to run your code.
The Assert, Deny, and PermitOnly stack walk modifiers do not affect the evaluation of link demands. Because link demands do not perform a stack walk, the stack walk modifiers have no effect on link demands.
If a method protected by a link demand is accessed through Reflection in the .NET Framework, then a link demand checks the immediate caller of the code accessed through reflection. This is true both for method discovery and for method invocation performed using reflection. For example, suppose code uses reflection to return a MethodInfo object representing a method protected by a link demand and then passes that MethodInfo object to some other code that uses the object to invoke the original method. In this case the link demand check occurs twice: once for the code that returns the MethodInfo object and once for the code that invokes it.
A link demand performed on a static class constructor does not protect the constructor because static constructors are called by the system, outside the application's code execution path. As a result, when a link demand is applied to an entire class, it cannot protect access to a static constructor, although it does protect the rest of the class.
The following code fragment declaratively specifies that any code linking to the ReadData method must have the CustomPermission permission. This permission is a hypothetical custom permission and does not exist in the .NET Framework. The demand is made by passing a SecurityAction.LinkDemand flag to the CustomPermissionAttribute.