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Code Access Security in the .NET Framework : Page 2

Code access security (CAS) is a new feature provided by the .NET Common Language Runtime. CAS is a security model that lets you grant or deny execution permissions to an assembly according to its "properties," called evidence, such as its strong name or publisher. CAS is completely orthogonal to classic security models that lie on the authentication-permissions mechanism based on the identity of the caller. This article is a concise introduction to this compelling and fascinating topic.


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Permissions Sets, Code Groups, and Code Access Policies
Code groups are the building blocks of security policies. A Code Group is made of an association between an evidence value, also known in this case as a membership condition, and a permission set (that gathers together a set of permissions).
For instance, a fictitious code group could be the WindowsUpdate code group defined as

URL=http://www.microsoft.com (evidence membership condition) / Everything (permission set).

A hierarchical structure of Code Groups defines a security policy. The .NET framework comes with three different security policies: Enterprise, Machine, User. Additionally an host can define application domain-level policy by calling the AppDomain.SetAppDomainPolicy method on the System.AppDomain class. The first three policies are typically set by administrator while the latter is eventually defined by developers.

The .NET framework comes with a number of built-in permission sets as shown below.

  • FullTrust
  • SkipVerification
  • Execution
  • Nothing
  • LocalIntranet
  • Internet
  • Everything


You can define additional custom permission sets if necessary gathering together the desired code access permissions listed in the previous paragraph. Note that you cannot assign directly a list of permission to a membership condition to define a Code Group.

Custom permission sets are defined in XML files and then loaded in the desired policy.

The code permissions an assembly is granted is made of the intersection of the permissions each policy assign to the assembly (unless the -Levelfinal attribute is specified). The order of policy level evaluation for the LevelFinal attribute is enterprise, machine, and then user. The application domain policy level, if present, is always intersected with other levels even if the LevelFinal attribute is in effect for one of those levels.

Now there is just thing left to explain: How a policy is applied to a given assembly, that is, how each policy, separately, builds up the list of permissions sets an assembly is granted. The procedure is as follows:
 

  1. CAS checks if the assembly can be granted membership to the root Code Group (this check always succeeds in a default configuration since the root Code Group is defined as membership: All Code Permission set: Nothing)
  2. CAS then evaluates if the assembly can be granted membership to the root’s child Code Groups (matching the assembly evidence with the Code Group membership condition). For any granted membership the corresponding permissions sets are added.
  3. CAS proceeds evaluating the membership of the assembly against the Code Groups that are child of a Code Group the assembly has been granted membership in the previous step. CAS ignores Code Groups sub-trees that are child of Code Groups that have not been granted membership in the previous step.
  4. Back to step 3 till all paths have been covered.
     

This logic is described graphically in the picture below, where green blocks represent granted membership; red blocks represent denied membership and yellow ones are code groups that are not evaluated.
 

Code groups, permission sets and policy structures can be queried and modified using the caspol.exe command line tool or using a much more friendly tool called the Microsoft .NET Framework Configuration Management Console snap-in.
Here I’ve included a snapshot of the CAS entities as shown by the snap-in
 

 

Code Access Permission Requests
CAS let’s you declare via a set of attributes at assembly level what permissions (or permissions sets) your code

  • needs at minimum
  • optionally needs
  • refuses


thus interacting/modifying the permissions assigned by the evidence vs. security policy mechanism described in the previous chapters.
In this code snippet a VB.NET assembly refuses the permissions associated with the FullTrust permission set and requires the File I/O permission.

<assembly: FileIOPermission(SecurityAction.RequestMinimum, Unrestricted := 
True)>
<assembly: PermissionSetAttribute(SecurityAction.RequestRefuse, Name := 
"FullTrust")>
When the CLR loads the assembly it evaluates the assembly permission requests against the permissions that have been assigned to it by CAS policies. If the assembly has not been granted the RequestMinimum permissions the assembly won’t load and an exception will be thrown, additionally the CLR will, eventually, remove permissions that have been granted to the assembly due its evidence but that have been explicitly refused by RequestRefuse declarations.

CAS Requests are not compulsory, still there are number of reason why you would won’t to evaluate to use them: the main reason is to avoid having to assembly loaded if the permissions restrictions would make its functionalities unacceptably limited.
Moreover, when declaring CAS requests your code is not required to gracefully handle any and all situations where not being granted some permission might prevent it from executing properly.
One last advantage is that putting permissions requests into the assembly metadata let administrators easily explore the assembly requirements with the Permission View tool (Permview.exe) to adjust, eventually, security policies.





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