Improving Web Application Security: Threats and - CGISecurity

582 Part IV: Securing Your Network, Host, and ApplicationIf a fixed impersonation account is provided through the userName and passwordattributes, ASP.NET uses that account instead of the IIS UNC token to access theshare. Any resource access performed by your application also uses the fixedimpersonation account.Note In the above example, Aspnet_setreg.exe has been used to store the encrypted accountcredentials in the registry.If you enable impersonation of the original caller (IIS authenticated identity) by usingthe following configuration, ASP.NET still uses the UNC-provided token to accessyour application’s files on the share, although any resource access performed by yourapplication uses the impersonation token.Note The account used for the UNC share must also be able to read Machine.config.Code Access Security ConsiderationsApplications on a UNC share are granted the intranet permission set by code accesssecurity policy. The intranet permission set does not containAspNetHostingPermission, which ASP.NET Web applications require to run, so yourapplication will not run without explicit policy modifications.You have two options:● Grant full trust to the UNC share on which your application is hosted.This is the simplest option to manage and if you run .NET Framework version 1.0,this is the only option because ASP.NET version 1.0 Web applications require fulltrust.● Configure code access security policy to grant your code theAspNetHostingPermission and any other permission it might require based onthe types of resources it accesses and the operations it performs.Because of the way in which ASP.NET dynamically creates code and compilespage classes, you must use a code group for the UNC and the Temporary ASP.NETFiles directory when you configure policy. The default temporary directory is\WINNT\Microsoft.NET\Framework\{version}\Temporary ASP.NET Files, butthe location is configurable on a per application basis by using the tempDirectoryattribute of the element.For more information about ASP.NET code access security policy and sandboxingprivileged code, see Chapter 9, “Using Code Access Security with ASP.NET.”Note When configuring policy, you should grant trust to the share (by using a file location) ratherthan to the zone. This provides finer granularity because you do not affect all the applications ina particular zone.