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NET brings with it a new security model for deployed code Instead of an assembly getting the permissions of the process running the code, the NET Code Access Security (CAS) model grants code permissions based on where the code originates[2] To view the current permission settings on your machine, use the Microsoft NET Framework Configuration tool (available in your Administration Tools menu) Drilling into the Permission Sets for the Machine's Runtime Security Policy reveals a number of entries, including FullTrust, LocalIntranet, Internet, and so on Figure 153 shows the set of Internet permissions
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For a much more detailed discussion of CAS, you'll want to read Essential NET (Addison-Wesley, 2003), by Don Box, with Chris Sells
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Figure 153 Default Internet Permission Set
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Table 151 compares the LocalIntranet permission set to the Internet permission set Assemblies are associated with a permission set in any number of ways: according to the publisher, the site, the strong name, the security zone, and so on Most of the default code groups associate code with a zone For example, the My_Computer_Zone is associated with the FullTrust permission set, and the Local_Intranet_Zone is associated with the LocalIntranet permission set In release 10 of NET, the Internet_Zone was associated with the Internet permission set, but as of service pack 1 of the NET runtime, code from the Internet_Zone is associated with the Nothing permission set by default This change in SP1 reflects some doubt that Microsoft had as to the full safety of the NET Framework Class Library code As of NET 11, the doubts have been resolved, and the Internet_Zone is again associated with the Internet permission set
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Table 151 Intranet and Internet CAS Permissions
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Permission FileDialog FileDialog IsolatedStorageFile IsolatedStorageFile Printing Printing Reflection Security Security UI UI UI UI Web Web Unrestricted Access=Open Allow=AssemblyIsolationByUser Allow=DomainIsolationByUser Level=DefaultPrinting Level=SafePrinting Flags=ReflectionEmit Flags=Assertion Flags=Execution Unrestricted Clipboard=OwnClipboard Window=SafeSubWindows Window=SafeTopLevelWindows Connect=http to originating site Connect=https to originating site Level LocalIntranet Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Internet No Yes No Yes No Yes No No Yes No Yes Yes Yes Yes Yes
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The zone where an assembly originates is determined by the path used to find the assembly, as shown in Table 152 If an assembly needs to know the zone it's running in, it can access the zone via the Zone class in SystemSecurityPolicy: using SystemSecurity; using SystemSecurityPolicy; string appbase = AppDomainCurrentDomainBaseDirectory; SecurityZone zone = ZoneCreateFromUrl(appbase)SecurityZone; By default, the loaded assembly gets the union of all the permissions from all the code groups to which it belongs and must live within the confines of those permissions Any attempt to perform an action for which the assembly does not have the corresponding permission will result in a security exception
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Checking for Permissions
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Although an application can catch a SecurityException if it violates its set of permissions, it can also check first to see whether it's got the permissions it's after This allows an application to downgrade its capabilities if appropriate permissions aren't available Checking for permission is a matter of creating a permission object, demanding that permission, and catching the security exception if the demand fails
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Table 152 How an Assembly's Zone Is Determined
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Path Local file UNC name or non-dotted site URL All numeric IP address or dotted-site URL c:\ foo\fooexe \\server\foo\fooexe or http://server/foo/fooexe or http://localhost/foo/fooexe or z:\foo\fooexe if z is mapped to a network share http://1115768663/foo/fooexe or http://wwwsellsbrotherscom/foo/fooexe or http://127001/foo/fooexe Example Zone MyComputer LocalIntranet
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Internet
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For example, to check whether your control is allowed to fire an event into unmanaged code, you use an instance of the SecurityPermission class from the SystemSecurityPermissions namespace: using SystemSecurity; using SystemSecurityPermissions; bool HavePermission(IPermission perm) { try { permDemand(); } catch( SecurityException ) { return false; } return true; } void label1_Click(object sender, EventArgs e) { SecurityPermissionFlag flag = SecurityPermissionFlagUnmanagedCode; SecurityPermission perm = new SecurityPermission(flag); if( !HavePermission(perm) ) return; // Fire event } If you wonder which permission you need for a specific call, you should start with the security exception itself Unfortunately, unlike most other exceptions in NET, the security exception provides somewhat terse information That's to prevent bad people from learning too much about an application's implementation, although it does tend to make debugging security exceptions a bit harder In these cases, I check the documentation, which is surprisingly good about telling you which permissions are needed and when[3]
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To check on all kinds of details, including permissions, for a WinForms control hosted in IE, you might try adjusting the undocumented DebugIEHost Registry setting as discussed at http://discussdevelopcom/archives/waexe A2=ind0109A&L=DOTNET&P=R9256
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