// Perform tasks that require user permissions in .NET

Drawing QR Code in .NET // Perform tasks that require user permissions

// Perform tasks that require user permissions
Creating QR In .NET Framework
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Reading QR In .NET
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// Undo the impersonation, reverting to the previous user context
Make Bar Code In .NET
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Read Bar Code In .NET Framework
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realUser.Undo();
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Drawing QR Code In .NET
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// Displays NT AUTHORITY\NETWORK SERVICE" in user3Label
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user3Label.Text = WindowsIdentity.GetCurrent().Name;
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Lesson 2: Controlling Authorization in ASP.NET Applications
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Encode OneCode In .NET
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Paint EAN 13 In Visual Basic .NET
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USS Code 128 Scanner In .NET
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Imports System.Security.Principal
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Barcode Reader In .NET Framework
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Displays NT AUTHORITY\NETWORK SERVICE" in user1Label
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user1Label.Text = WindowsIdentity.GetCurrent.Name
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Impersonate the user with the account they used to authenticate
Dim realUser As WindowsImpersonationContext
realUser = CType(User.Identity, WindowsIdentity).Impersonate
Displays domain and username of authenticated user in user2Label.
For example, CONTOSO-DEV\Administrator"
user2Label.Text = WindowsIdentity.GetCurrent.Name
Perform tasks that require user permissions Undo the impersonation, reverting to the previous user context
realUser.Undo
Displays NT AUTHORITY\NETWORK SERVICE" in user3Label
user3Label.Text = WindowsIdentity.GetCurrent.Name
To use this technique effectively, leave ASP.NET impersonation disabled but require Windows authentication. Create the WindowsImpersonationContext object, and call the WindowsIdentity.Impersonate method immediately before performing a task that requires privileges that the Network Service account lacks, such as making an update to a database. After the task has been completed, call the WindowsImpersonationContext.Undo method to return to the Network Service security context.
How to Use Code Access Security to Limit Privileges
You can control the level of trust granted to external applications that your ASP.NET Web application might call. By default, trust is not a factor for ASP.NET applications, because the Machine.config file is preconfigured to give full trust to ASP.NET applications. This snippet from the Machine.config file shows the default settings of the <securityPolicy> and <trust> sections:
<securityPolicy> <trustLevel name="Full" policyFile="internal"/> <trustLevel name="High" policyFile="web_hightrust.config"/> <trustLevel name="Medium" policyFile="web_mediumtrust.config"/> <trustLevel name="Low" policyFile="web_lowtrust.config"/> <trustLevel name="Minimal" policyFile="web_minimaltrust.config"/> </securityPolicy> <trust level="Full" originUrl=""/>
The <securityPolicy> section defines the different levels of trust that might be specified for ASP.NET applications. Each <trustLevel> subsection defines a unique level of trust and has two attributes: name and policyFile. The name attribute gives the trust level a friendly name that will be referenced in the <trust> section, and the policyFile
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9
Hardening ASP.NET Applications
attribute references another configuration file (located in the same folder containing the Machine.config file) that contains the details of that level s trust settings. By default, five different levels of trust exist: Full, High, Low, None, and Minimal. The Full trust level does not have a configuration file, because it causes the runtime to simply skip all code access security (CAS) checks. The <trust> section defines the level of trust that ASP.NET applications run from a remote URL will receive. The default setting in the Machine.config file grants all applications Full trust. To configure an application run from http://remoteapp/appdir/ to run with a Low level of trust, add this line to the Web.config file:
<trust level="Low" originUrl="http://remoteapp/appdir/" />
How to Restrict User Access by Using IP Addresses
A common technique for restricting authorization to Web applications is to verify a user s IP address. You can use one of two techniques to restrict authorization based on source IP address: configure restrictions in IIS or check the source IP address from within your ASP.NET application. The more efficient way to control access based on source IP address is to add IP address restrictions within IIS. Access that is restricted within IIS can be updated easily by systems administrators. Figure 9-4 shows IIS configured to allow only requests from the 192.168.1.0 subnet.
F09NS04
Figure 9-4 IIS can restrict access based on IP address, network ID, and domain name.
Alternatively, you can restrict access within your ASP.NET code by checking the user s source IP address. The user s IP address is contained in the Request.UserHostAddress property, and you can verify all or portions of the user s IP address by using regular expressions. If you choose to control authorization based on the IP address, you must enable administrators to specify the IP addresses and networks in your application s configuration. Even if your application is for internal use, administrators must be able to renumber networks without modifying your application s source code.
Lesson 2: Controlling Authorization in ASP.NET Applications
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Tip Source IP filtering should not be your only authorization technique, because source IP addresses can be spoofed by skilled attackers with direct access to your network. However, source IP filtering can be a useful part of a defense-in-depth approach.
How IIS and ASP.NET Handle File Extensions
IIS authentication, when enabled, authenticates all requests for a folder regardless of the type of file being retrieved. ASP.NET authentication works very differently, however, and can authenticate only those requests that IIS passes to the .NET Framework. By default, IIS 5.0 with the .NET Framework installed is configured to pass requests for files ending in the extensions .asax, .ascx, .ashx, .asmx, .aspx, .axd, .config, .cs, .csproj, .java, .jsl, .licx, .rem, .soap, .vb, .vbproj, .webinfo, .resx, .resources, .vjsproj, and .vsdisco. This means that requests for any other file will not be passed through ASP.NET, and therefore cannot be controlled by ASP.NET authentication. Here is a practical example: Your company uses a public Web site to sell recipes written using Adobe Acrobat. These .PDF files are located in the /recipes/ virtual directory. Users should be able to download these recipes only after they pay a fee and the application assigns them a unique user name and password. You created an ASP.NET application that uses Forms authentication to verify users, and you configured the /recipes/ virtual directory to require Forms authentication by setting the <authentication> section of the virtual directory s Web.config file appropriately. However, after the first user purchases a recipe, he e-mails the URL of the .pdf file to his friends, and other people are able to download the file directly without being authenticated. This problem occurs because files with extensions that are not associated with the aspnet_isapi.dll file are processed directly by IIS, and therefore cannot trigger an ASP.NET authentication event. The .pdf extension is not one of the extensions mapped to ASP.NET; therefore, IIS retrieves the file from the context of its anonymous user account and sends the file to the user. To overcome this, add .pdf to the IIS extension mappings as described in the next section. After this change is made, IIS will send all requests for files ending in .pdf to ASP.NET. ASP.NET will enforce authentication rules and redirect users requesting the file to the logon page. To ensure the security of a virtual directory authenticated with ASP.NET, map all file extensions of documents that should be protected to the aspnet_isapi.dll file. The following sections show how to configure additional ASP.NET file types in IIS 6.0 and .config files.
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