Improving Web Application Security: Threats and - CGISecurity

252 Part III: Building Secure Web ApplicationsSummaryCode access security is a resource constraint security model that can be used to helpprovide application isolation. Applications can be configured to run at variouspartial-trust levels. The trust level determines the permissions that are granted to theASP.NET Web application or Web service. This determines the resource types that canbe accessed, and the other types of privileged operation that can be performed. Notethat all resource access is ultimately subject to operating system security.The recommended isolation model uses IIS 6.0 application pools on WindowsServer 2003 and provides process level isolation in addition to code access security.On Windows 2000, isolation can only be achieved using code access security andseparate thread identities.Migrating an application to run with partial trust usually requires a certain amount ofreengineering. You might need to reengineer if the application accesses resources thatare not permitted by the partial trust level or if it calls strong named assemblies thatdo not contain APTCA. In these cases, you can sandbox privileged resource access inseparate wrapper assemblies. In some scenarios, you might be able to create and usecustom policy files, although this depends on your Web server’s security policy.It is a good design practice to place resource access code in separate assemblies andavoid placing this code in .aspx files and code behind files. The use of separateassemblies allows code access security policy to be applied to the assemblyindependently from the Web application and it allows you to develop sandboxedtrusted code to perform resource access.Additional ResourcesFor more information, see the following resources:●●●●“Security in .NET: The Security Infrastructure of the CLR Provides Evidence,Policy, Permissions, and Enforcement Services” in MSDN Magazine athttp://msdn.microsoft.com/msdnmag/issues/02/09/SecurityinNET/default.aspx.“Security in .NET: Enforce Code Access Rights with the Common LanguageRuntime” in MSDN Magazine at http://msdn.microsoft.com/msdnmag/issues/01/02/CAS/default.aspx.LaMacchia, Lange, Lyons, Martin, and Price. .NET Framework Security. AddisonWesley Professional, 2002.“How To: Create a Custom Encryption Permission” in the “How To” section ofthis guide.