Improving Web Application Security: Threats and - CGISecurity

Chapter 8: Code Access Security in Practice 203For example:[SecurityPermission(SecurityAction.LinkDemand,Flags=SecurityPermissionFlag.ControlPrincipal)]public struct SomeStruct{// This explicit constructor is protected by the link demandpublic SomeStruct(int i){field = i;}private int field;}The following two lines of code both result in a new structure with the fieldinitialized to zero. However, only the first line that uses the explicit constructor issubject to a link demand.SomeStruct s = new SomeStruct(0);SomeStruct s = new SomeStruct();The second line is not subject to a link demand because a default constructor is notgenerated. If this were a class instead of a structure, the compiler would generate adefault constructor annotated with the specified link demand.Virtual Methods and Link DemandsIf you use link demand to protect a method override in a derived class, make sureyou also put it on the corresponding virtual base class method. Otherwise, if the JITcompiler sees a reference to the base class type where no link demand is present, nolink demand is performed.Assert and RevertAssertYou can call the CodeAccessPermission.Assert method to prevent a demandpropagating beyond the current stack frame. By using Assert, you vouch for thetrustworthiness of your code’s callers. Because of the potential for luring attacks,Assert needs to be used with caution.Asserts are most often used to sandbox privileged code. If you develop code that callsAssert, you need to ensure that there are alternate security measures in place toauthorize the calling code. The following recommendations help you to minimize therisks.● Use the demand / assert pattern● Reduce the Assert duration