Improving Web Application Security: Threats and - CGISecurity

Chapter 8: Code Access Security in Practice 201Only use link demands in an assembly when you trust the assembly’s callers not toexpose its functionality further (for example, when the caller is an application, not alibrary) or when you know it is safe just to verify the immediate caller’s identity withan identity permission demand.Performance and Link DemandsCompared to other Web application performance issues such as network latency anddatabase access, the cost of a stack walk is small. Do not use link demands purely forperformance reasons. Full demands provide a much greater degree of security.Calling Methods with Link DemandsIf you call a link demand protected method, only your code will be checked by thelink demand. In this situation, you should make sure your code takes adequatemeasures to authorize its callers, for example, by demanding a permission.Mixing Class and Method Level Link DemandsMethod level link demands override class level link demands. For example, in thefollowing code fragment, the link demand for FileIOPermission must be repeated onthe method declaration or the EnvironmentPermission link demand replaces theclass level FileIOPermission demand.[FileIOPermission(SecurityAction.LinkDemand, Unrestricted=true)]public sealed class SomeClass{// The unrestricted FileIOPermission link demand must be restated at the// method level, if the method is decorated with another link demand.// Failure to do so means that (in this example) that the// EnvironmentPermission link demand would override the class level// FileIOPermission link demand[FileIOPermission(SecurityAction.LinkDemand, Unrestricted=true)][EnvironmentPermission(SecurityAction.LinkDemand, Read="PATH")]public void SomeMethod(){}}