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Best Practice for Handling Errors
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Use PermitOnly to limit permissions during error-handling routines. Attackers often initiate an error condition in an application and then abuse that error condition to perform tasks that would not be possible under normal circumstances. Using PermitOnly
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Application Security
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to limit CAS permissions to the bare minimum required to log the event and report an error to the user significantly reduces the risk that your error-handling routine can be abused. If your application will continue running after the error, be sure to revert to your original permissions otherwise, normal application functionality will not be available. For example, the following code catches an exception, restricts CAS permissions to those required to add events, and then reverts to the previous permission set:
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' VB Try ' Assembly logic Catch Dim errorPerms As EventLogPermission = _ New EventLogPermission (PermissionState.Unrestricted) errorPerms.PermitOnly ' Log event CodeAccessPermission.RevertPermitOnly End Try // C# try { // Assembly logic } catch { EventLogPermission errorPerms = new EventLogPermission(PermissionState.Unrestricted); errorPerms.PermitOnly(); // Log event CodeAccessPermission.RevertPermitOnly(); }
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Restricting permissions to those required for a specific block of code is an excellent example of following the principle of least privilege. Although it s particularly important during error-catching routines, you can use this technique to limit the permissions of any block of code.
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How to Relax Permissions and Potentially Improve Performance
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Using CAS demands improves the security of an assembly but can decrease performance. In particular, calling a permission s Demand method is costly because it forces the runtime to systematically check the permission of every caller. LinkDemand, discussed earlier, is one way to improve upon the performance of the Demand method, but it sacrifices some level of security. Another technique is the Assert method, which causes the runtime to bypass any security checks.
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Lesson 3: Using Declarative and Imperative Security to Protect Methods
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IMPORTANT Compared with assert in C++
CodeAccessPermission.Assert is nothing like the assert function in C or C++.
Permission objects include the Assert method to enable a method to vouch for all callers. Figure 11-11 shows how a call to Assert stops the runtime from checking the CAS permissions of assemblies higher in the stack. This has two effects: improving performance by reducing the number of permission checks and allowing underprivileged code to call methods with higher CAS permission requirements.
Assembly 1
Method1 1 Assembly1.Method1 calls Class2.Method2
Assembly1.Method1 is not checked, because Method2 s Assert() vouched for Assembly1
Class2
Method2 permission.Assert()
Class2.Method2 calls Class3.Method3 Class3
Runtime checks permissions of Class2.Method2, finds the Assert(), and stops checking 3
Method3
Class3.Method3 makes a SecurityAction.Demand check 2
Figure 11-11 Assert blocks demand checks, increasing performance and allowing underprivileged code to call methods with CAS permission requirements
11
Application Security
For example, if you create a RegistryPermission object and call the Assert method, your assembly must be granted RegistryPermission, but any code calling your assembly does not require the permission. If you call another method that uses Demand to require RegistryPermission, Demand will succeed whether or not your caller has been granted RegistryPermission. You can use Assert either declaratively or imperatively, and the syntax is identical to other types of CAS declarations. The following example asserts permissions declaratively:
' VB <FileIOPermissionAttribute(SecurityAction.Assert, _ ViewAndModify := "C:\Program Files\")> _ <WebPermission(SecurityAction.Assert, _ ConnectPattern:="http://www\.microsoft\.com/.*")> _ // C# [FileIOPermission(SecurityAction.Assert, ViewAndModify = @"C:\Windows\")] [WebPermission(SecurityAction.Assert, ConnectPattern = @"http://www\.microsoft\.com/.*")]
Although the following example asserts permissions imperatively:
' VB ' Block all CAS permission checks for file access to the Windows directory Dim filePermissions As FileIOPermission = _ New FileIOPermission(FileIOPermissionAccess.AllAccess, "C:\Windows\") filePermissions.Assert() ' Method logic ' Block all CAS permission checks for Web access to www.microsoft.com Dim connectPattern As Regex = New Regex("http://www\.microsoft\.com/.*") Dim webPermissions As WebPermission = _ New WebPermission(NetworkAccess.Connect, connectPattern) webPermissions.Assert() ' Method logic // C# // Block all CAS permission checks for file access to the Windows directory FileIOPermission filePermissions = new FileIOPermission(FileIOPermissionAccess.AllAccess, @"C:\Windows\"); filePermissions.Assert(); // Method logic // Block all CAS permission checks for Web access to www.microsoft.com Regex connectPattern = new Regex(@"http://www\.microsoft\.com/.*"); WebPermission webPermissions = new WebPermission(NetworkAccess.Connect, connectPattern); webPermissions.Assert(); // Method logic
Lesson 3: Using Declarative and Imperative Security to Protect Methods
To successfully use Assert, the assembly must have the SecurityPermissionFlag.Assertion privilege as well as the privilege being asserted. In the .NET Framework Configuration tool, SecurityPermissionFlag.Assertion is represented by the Assert Any Permission That Has Been Granted item in the Security permission properties dialog box. The FullTrust, LocalIntranet, and Everything permission sets have this permission. Using Assert allows an assembly to vouch for the security of lesser-privileged assemblies. This is an excellent way to grant additional functionality to assemblies that would normally lack CAS permissions. For example, you can use an Assert to allow an assembly in the Internet zone to save a file to the user s disk. Simply create an assembly with the AllowPartiallyTrustedCallersAttribute. Then create a public method that writes the file, create a FileIOPermission object, and call the Assert method before writing the file. The assembly in the Internet zone can save a file to a user s disk without requiring the administrators to grant file permissions to the Internet zone. To decrease the opportunity for an attacker to abuse asserted permissions, use the CodeAccessPermission.RevertAssert static method. As the name suggests, calling this method erases the assertion and returns CAS permission checking to the normal state. Use a try/finally block to ensure that you call RevertAssert after every Assert, even if a failure occurs. The following method demonstrates this and is also an excellent example of how to fail to a more secure permission set:
' VB Dim filePermissions As FileIOPermission = _ New FileIOPermission (FileIOPermissionAccess.Write, _ "C:\Inetpub\NewFile.txt") filePermissions.Assert Try Dim newFile As StreamWriter = New StreamWriter _ ("C:\Inetpub\NewFile.txt") newFile.WriteLine("Lesser privileged applications can save a file.") newFile.Close Finally CodeAccessPermission.RevertAssert End Try // C# FileIOPermission filePermissions = new FileIOPermission(FileIOPermissionAccess.Write, @"C:\Inetpub\"); filePermissions.Assert(); try { StreamWriter newFile = new StreamWriter(@"C:\Inetpub\NewFile.txt"); newFile.WriteLine("Lesser privileged applications can save a file."); newFile.Close(); }
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