Improving Web Application Security: Threats and - CGISecurity

Chapter 8: Code Access Security in Practice 183CodeAll managed code is subject to code access security. When an assembly is loaded, it isgranted a set of code access permissions that determines what resource types it canaccess and what types of privileged operations it can perform. TheMicrosoft .NET Framework security system uses evidence to authenticate (identify)code to grant permissions.Note An assembly is the unit of configuration and trust for code access security. All code in thesame assembly receives the same permission grant and is therefore equally trusted.EvidenceEvidence is used by the .NET Framework security system to identify assemblies.Code access security policy uses evidence to help grant the right permissions to theright assembly. Location-related evidence includes:● URL. The URL that the assembly was obtained from. This is the codebase URL inits raw form, for example, http://webserver/vdir/bin/assembly.dll orfile://C:/directory1/directory2/assembly.dll.● Site. The site the assembly was obtained from, for example, http://webserver. Thesite is derived from the codebase URL.● Application directory. The base directory for the running application.●Zone. The zone the assembly was obtained from, for example, LocalIntranet orInternet. The zone is also derived from the codebase URL.Author-related evidence includes:●●Strong name. This applies to assemblies with a strong name. Strong names are oneway to digitally sign an assembly using a private key.Publisher. The Authenticode signature; based on the X.509 certificate used to signcode, representing the development organization.Important Publisher evidence (the Authenticode signature) is ignored by the ASP.NET host andtherefore cannot be used to configure code access security policy for server-side Webapplications. This evidence is primarily used by the Internet Explorer host.●Hash. The assembly hash is based on the overall content of the assembly andallows you to detect a particular compilation of a piece of code, independent ofversion number. This is useful for detecting when third party assemblies change(without an updated version number) and you have not tested and authorizedtheir use for your build.