Improving Web Application Security: Threats and - CGISecurity

Chapter 9: Using Code Access Security with ASP.NET 231Developing Partial Trust Web ApplicationsPartial trust Web applications are applications that do not have full trust and have arestricted set of code access permissions determined by code access security policy.As a result, partial-trust applications are limited in their ability to access securedresources and perform other privileged operations. Certain permissions are denied topartial-trust applications, so resources requiring those permissions cannot be directlyaccessed. Other permissions are granted in a restricted way, so resources that requirethose permissions might be accessible, but in a limited way. For example, a restrictedFileIOPermission might specify that the application can access the file system, butonly in directories beneath the application’s virtual directory root.Why Partial Trust?By configuring a Web application or Web service for partial trust, you can restrict theapplication’s ability to access crucial system resources or resources that belong toother Web applications. By granting only the permissions that the applicationrequires and no more, you can build least privileged Web applications and limitdamage potential should the Web application be compromised by a code injectionattack.Problems You Might EncounterIf you take an existing Web application and reconfigure it to run at a partial-trustlevel, you are likely to run into the following issues, unless the application isextremely limited in the resources it accesses:● Your application is unable to call strong named assemblies that are not annotatedwith AllowPartiallyTrustedCallersAttribute (APTCA). Without APTCA, strongnamed assemblies issue a demand for full trust, which will fail when the demandreaches your partial-trust Web application. Many system assemblies only supportfull-trust callers. The following list shows which .NET Framework assembliessupport partial-trust callers and can be called directly by partial-trust Webapplications without necessitating sandboxed wrapper assemblies.Note Sandboxing is discussed in detail later in this chapter.The following system assemblies have APTCA applied, which means that they canbe called by partial-trust Web applications or any partially trusted code:●●●●●System.Windows.Forms.dllSystem.Drawing.dllSystem.dllMscorlib.dllIEExecRemote.dll