Code Access Security

This installment of .NET Nuts & Bolts is part one of a two-part series exploring code access security and how it is controlled by the Microsoft .NET Framework. The Microsoft .NET Framework includes a number of security features that assist you in developing secure applications. The security system, which is a fundamental part of the common language runtime (CLR), controls execution of .NET code. It includes handy features such as the following:Type safety enforcement, which eliminates the potential for buffer overruns Arithmetic error trapping, which detects the potential for underflows and overflows In addition, the .NET Framework provides the concept of evidence-based security. Evidence-based security works on top of the security provided by the operating system. For example, it works on top of Win32 security but is not a replacement for Win32 security. While Win32 is based on the user, the evidence-based security is based on the assembly. It gathers and presents information (or evidence) about the assembly to the security system, which then determines whether or not to allow the code to execute. For example, if code tries to read a file during execution, the security system verifies that the assembly has the required permissions and either grants access or throws a SecurityException.Evidence about an assembly can be controlled and influenced through things like strongly named assemblies, Authenticode signatures, or other custom information. Evidence is mapped to permissions through security policies, which rely on permission sets, code groups, and policy levels (enterprise, machine, and user settings) to achieve the mapping. Policies can be deployed throughout your organization through the Active Directory, but this discussion doesn't get into the specifics of that.

.NET has two kinds of security:

1. Role Based Security
2. Code Access Security

The Common Language Runtime (CLR) allows code to perform only those operations that the code has permission to perform. So CAS is the CLR's security system that enforces security policies by preventing unauthorized access to protected resources and operations. Using the Code Access Security, you can do the following:

1. Restrict what your code can do
2. Restrict which code can call your code
3. Identify code

Code Access Security consists of following item

1. Permissions
2. Permissions Sets
3. Code Groups
4. Evidence
5. Policy

Permissions

Permissions represent access to a protected resource or the ability to perform a protected operation. The .NET Framework provides several permission classes, like FileIOPermission (when working with files), UIPermission (permission to use a user interface), SecurityPermission (this is needed to execute the code and can be even used to bypass security) etc.

Permission sets
A permission set is a collection of permissions. You can put FileIOPermission and UIPermission into your own permission set and call it "My_PermissionSet". A permission set can include any number of permissions. FullTrust, LocalIntranet, Internet, Execution and Nothing are some of the built in permission sets in .NET Framework. FullTrust has all the permissions in the world, while Nothing has no permissions at all, not even the right to execute.

Code groups
Code group is a logical grouping of code that has a specified condition for membership. Code from http://www.somewebsite.com/ can belong to one code group, code containing a specific strong name can belong to another code group and code from a specific assembly can belong to another code group. There are built-in code groups like My_Computer_Zone, LocalIntranet_Zone, Internet_Zone etc. Like permission sets, we can create code groups to meet our requirements based on the evidence provided by .NET Framework. Site, Strong Name, Zone, URL are some of the types of evidence.

Policy
Security policy is the configurable set of rules that the CLR follows when determining the permissions to grant to code. There are four policy levels - Enterprise, Machine, User and Application Domain, each operating independently from each other. Each level has its own code groups and permission sets.