Code Access Security

What is Code Access Security (CAS)?

CAS is the part of the .NET security model that determines whether or not code is allowed to run, and what resources it can use when it is running. For example, it is CAS that will prevent a .NET web applet from formatting your hard disk.

How does CAS work?

The CAS security policy revolves around two key concepts - code groups and permissions. Each .NET assembly is a member of a particular code group, and each code group is granted the permissions specified in a named permission set.
For example, using the default security policy, a control downloaded from a web site belongs to the 'Zone - Internet' code group, which adheres to the permissions defined by the 'Internet' named permission set. (Naturally the 'Internet' named permission set represents a very restrictive range of permissions.)

Who defines the CAS code groups?

Microsoft defines some default ones, but you can modify these and even create your own. To see the code groups defined on your system, run 'caspol -lg' from the command-line. On my system it looks like this:

Level = Machine
Code Groups:
1. All code: Nothing
1.1. Zone - MyComputer: FullTrust
1.1.1. Honor SkipVerification requests: SkipVerification
1.2. Zone - Intranet: LocalIntranet
1.3. Zone - Internet: Internet
1.4. Zone - Untrusted: Nothing
1.5. Zone - Trusted: Internet
1.6. StrongName -

0024000004800000940000000602000000240000525341

310004000003

000000CFCB3291AA715FE99D40D49040336F9056D7886

FED46775BC7BB5430BA4444FEF8348EBD06

F962F39776AE4DC3B7B04A7FE6F49F25F740423EBF2C0B

89698D8D08AC48D69CED0FC8F83B465E08

07AC11EC1DCC7D054E807A43336DDE408A5393A485561

23272CEEEE72F1660B71927D38561AABF5C

AC1DF1734633C602F8F2D5: Everything

Note the hierarchy of code groups - the top of the hierarchy is the most general ('All code'), which is then sub-divided into several groups, each of which in turn can be sub-divided. Also note that (somewhat counter-intuitively) a sub-group can be associated with a more permissive permission set than its parent.

How do I define my own code group?

Use caspol. For example, suppose you trust code from www.mydomain.com and you want it have full access to your system, but you want to keep the default restrictions for all other internet sites. To achieve this, you would add a new code group as a sub-group of the 'Zone - Internet' group, like this:

caspol -ag 1.3 -site www.mydomain.com FullTrust

Now if you run caspol -lg you will see that the new group has been added as group 1.3.1:

... 1.3. Zone - Internet: Internet

1.3.1. Site - www.mydomain.com: FullTrust

...

Note that the numeric label (1.3.1) is just a caspol invention to make the code groups easy to manipulate from the command-line. The underlying runtime never sees it.

How do I change the permission set for a code group?

Use caspol. If you are the machine administrator, you can operate at the 'machine' level - which means not only that the changes you make become the default for the machine, but also that users cannot change the permissions to be more permissive. If you are a normal (non-admin) user you can still modify the permissions, but only to make them more restrictive. For example, to allow intranet code to do what it likes you might do this:

caspol -cg 1.2 FullTrust

Note that because this is more permissive than the default policy (on a

standard system), you should only do this at the machine level - doing it at the user level will have no effect.

Can I create my own permission set?

Yes. Use caspol -ap, specifying an XML file containing the permissions in the permission set. To save you some time, here is a sample file corresponding to the 'Everything' permission set - just edit to suit your needs. When you have edited the sample, add it to the range of available permission sets like this:
caspol -ap samplepermset.xmlThen, to apply the permission set to a code group, do something like this:

caspol -cg 1.3 SamplePermSet

(By default, 1.3 is the 'Internet' code group)

I'm having some trouble with CAS. How can I troubleshoot the problem?

Caspol has a couple of options that might help. First, you can ask caspol to tell you what code group an assembly belongs to, using caspol -rsg. Similarly, you can ask what permissions are being applied to a particular assembly using caspol -rsp.

I can't be bothered with CAS. Can I turn it off?
Yes, as long as you are an administrator. Just run:
caspol -s off