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Is CAS dead in .NET 4?

With all the changes in the security system of .NET 4, the question frequently arises “so, is CAS dead now?”.   One of the reasons that this question comes up so frequently, is that the term CAS in the .NET 1 security model was overloaded to refer to many different aspects of the security system:

  • CAS policy – policy levels, code groups, and of course our old friend caspol.exe
  • CAS enforcement – primarily the act of demanding and asserting permissions
  • CAS permissions – granted by CAS policy or a host to set the level of operations that an application can perform

I’ve talked in the past about the many problems with CAS policy over the years.  There are versioning problems.  The host doesn’t have control over the policy applied to the code it is hosting.  Enterprise administrators don’t have a good way to deploy CAS policy updates.  CAS policy caused managed applications to run differently from native applications, often in confusing and undesirable ways.  And of course, there’s the general complexity and difficulty of use (caspol is nobody’s favorite tool).

For these reasons, in v4 of the CLR, CAS policy has been deprecated and policy decisions are instead left entirely up to the host of an application.  However, the other security mechanisms that fell under the name CAS, which allow hosts to configure AppDomains to host sandboxed code and allow library authors to write a safe APTCA library exposing services to partial trust absolutely still exist and are supported.

For instance, when a host sets up a sandboxed AppDomain to run code in, it does this by figuring out what grant set should be given to an application and supplying that grant as a set of permissions – the exact same permissions that have been used since v1 of the .NET Framework.   Custom permissions can still be created by hosts or APTCA library authors to protect their libraries, and assemblies and AppDomains still receive permission objects in their grant sets.

Similarly, permissions demands are still alive and well, and are one of the most common ways that safe critical APIs in APTCA libraries will check to ensure that the sandbox they are running in supports a given operation.   For example, opening a file is a security safe critical operation which demands FileIOPermission to ensure that the host has setup the current sandbox with permission to access the requested file.

What does all of this mean in practice for things like ClickOnce and ASP.NET Medium Trust sandboxes?   Both ASP.NET and ClickOnce are hosts that setup sandboxes for partial trust code – which is a core scenario for the CLR that is still very much alive.  ASP.NET simply sets up an AppDomain with the Medium Trust permission set (or whichever other permission set has been configured for the site in question), and all of the application assemblies loaded into that domain will receive the partial trust permission set that ASP.NET configured.  If those applications try to open a file or do some other operation that is only allowed in certain sandboxes, a permission demand will be issued, and if that demand succeeds the operation will succeed.

Similarly, ClickOnce continues to work in the same way as it always had.  The ClickOnce runtime sets up an AppDomain with the permissions specified in the application’s manifest and the application will run in a sandbox with that permission set.   Safe critical APIs which issue demands outside of the application’s grant set will lead to security exceptions, while safe critical APIs that access resources allowed under the application’s grant set will work just like they used to.

In fact, the actual ClickOnce code really didn’t change very much at all for v4 security.  Since ClickOnce has always setup homogenous AppDomains dating back to its introduction in .NET 2.0, it has never had a dependency on CAS policy at runtime!

Even though we’ve moved away from CAS policy, the CLR still provides mechanisms for partially trusted code to be setup and run – and that’s something we’ve continued to invest in making a better and safer experience.  A lot of our work with security transparency in this release, for instance, was to make it safer for APTCA library authors to expose their code to partial trust.  The new SecAnnotate tool was designed exactly to help ensure that more libraries could be safely exposed in a partial trust sandbox.

Recently, I was having a discussion with our security MVPs about how the overload of the term CAS is causing “CAS is dead” confusion, Keith Brown remarked to me that he prefers to think of it along these lines:  .NET 4: Security just got a whole lot simpler.