Improving Web Application Security: Threats and - CGISecurity

Chapter 8: Code Access Security in Practice 205Using Policy Permission GrantsYou can configure code access security policy to grant a restricted permission set to aspecific assembly. This constrains its ability to access resources or perform otherprivileged operations. For more information, see “How To: Configure Code AccessSecurity Policy to Constrain an Assembly” in the “How To” section of this guide.Using Stack Walk ModifiersYou can use stack walk modifiers to ensure that only specific permissions areavailable to the code that you call. For example, you can useSecurityAction.PermitOnly to ensure that your method and any methods that arecalled only have a restricted permission set. The following example applies a veryrestrictive permission set. The code only has the permission to execute. It cannotaccess resources or perform other privileged operations.[SecurityPermissionAttribute(SecurityAction.PermitOnly,Flags=SecurityPermissionFlag.Execution)]public void SomeMethod(){// The current method and downstream can only execute. They cannot access// resources or perform other privileged operations.SomeOtherMethod();}File I/OThe following sections show you how to use code access security to constrain varioustypes of resource access including file I/O, event log, registry, data access, directoryservices, environment variables, Web services, and sockets.To be able to perform file I/O, your assembly must be granted the FileIOPermissionby code access security policy. If your code is granted the unrestrictedFileIOPermission, it can access files anywhere on the file system, subject to Windowssecurity. A restricted FileIOPermission can be used to constrain an assembly’s abilityto perform file I/O, for example, by specifying allowed access rights (read,read/write, and so on.)Constraining File I/O within your Application’s ContextA common requirement is to be able to restrict file I/O to specific directory locationssuch as your application’s directory hierarchy.Note If your Web application is configured for Medium trust, file access is automatically restrictedto the application’s virtual directory hierarchy. For more information, see Chapter 9, “Using CodeAccess Security with ASP.NET.”