Improving Web Application Security: Threats and - CGISecurity

Chapter 8: Code Access Security in Practice 185For more information about how to use link demands correctly, see the “LinkDemands” section later in this chapter.Assert, Deny, and PermitOnly MethodsCode access permission classes support the Assert, Deny, and PermitOnly methods.You can use these methods to alter the behavior of a permission demand stack walk.They are referred to as stack walk modifiers.A call to the Assert method causes the stack walk for a matching permission to stopat the site of the Assert call. This is most often used to sandbox privileged code. Formore information, see the “Assert and RevertAssert” section later in this chapter.A call to the Deny method fails any stack walk that reaches it with a matchingpermission. If you call some non-trusted code, you can use the Deny method toconstrain the capabilities of the code that you call.A call to the PermitOnly method fails any unmatching stack walk. Like the Denymethod, it tends to be used infrequently but it can be used to constrain the actions ofsome non-trusted code that you may call.PolicyCode access security policy is configured by administrators and it determines thepermissions granted to assemblies. Policy can be established at four levels:● Enterprise. Used to apply Enterprise-wide policy.●●●Machine. Used to apply machine-level policy.User. Used to apply per user policy.Application Domain. Used to configure the application domain into which anassembly is loaded.ASP.NET implements application domain policy to allow you to configure codeaccess security policy for Web applications and Web services. For moreinformation about ASP.NET application domain policy, see Chapter 9, “UsingCode Access Security with ASP.NET.”Policy settings are maintained in XML configuration files. The first three levels ofpolicy (Enterprise, Machine, and User) can be configured by using the .NETFramework Configuration tool, which is located in the Administrative Tools programgroup or the Caspol.exe command line utility. ASP.NET application domain levelpolicy must currently be edited with a text or XML-based editor.For more information about policy files and locations, see Chapter 19, “Securing YourASP.NET Application and Web Services.”