Improving Web Application Security: Threats and - CGISecurity

232 Part III: Building Secure Web Applications●●●●●●Accessibility.dllMicrosoft.VisualBasic.dllSystem.XML.dllSystem.Web.dllSystem.Web.Services.dll● System.Data.dllIf your partial-trust application fails because it calls a strong named assembly thatis not marked with APTCA, a generic SecurityException is generated. In thiscircumstance, the exception contains no additional information to indicate that thecall failed because of a failed demand for full trust.Permission demands might start to fail. The configured trust level might not grantthe necessary permission for your application to access a specific resource type.The following are some common scenarios where this could prove problematic:●●●Your application uses the event log or registry. Partial trust Web applications donot have the necessary permissions to access these system resources. If yourcode does so, a SecurityException will be generated.Your application uses the ADO.NET OLE DB data provider to access a datasource. The OLE DB data provider requires full-trust callers.Your application calls a Web service. Partial-trust Web applications have arestricted WebPermission, which affects the ability of the application to callWeb services located on remote sites.Trust LevelsIf you plan to migrate an existing application to a partial-trust level, a good approachis to reduce permissions incrementally so that you can see what parts of yourapplication break. For example, start by setting the trust level attribute to High, thenMedium, and so on. Ultimately, the trust level you should target depends on thedegree of restriction you want to place on the application. Use the following asguidance:● Applications configured for high, medium, low, or minimal trust will be unable tocall unmanaged code or serviced components, write to the event log, accessMessage Queuing queues, or access OLE DB data sources.●●●●Applications configured for high trust have unrestricted access to the file system.Applications configured for medium trust have restricted file system access. Theycan only access files in their own application directory hierarchy.Applications configured for low or minimal trust cannot access SQL Serverdatabases.Minimal trust applications cannot access any resources.