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Managing Client Protection : User Account Control (part 3) - UAC Virtualization, UAC and Startup Programs, Compatibility Problems with UAC

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4/28/2013 11:34:45 AM

5. UAC Virtualization

By default, UAC virtualizes requests for protected resources to provide compatibility with applications not developed for UAC. This is important, because many applications written for Windows XP and earlier operating systems assume that the user has administrative privileges and attempt to write to protected resources such as the Program Files or System folders.

UAC virtualization redirects requests for the following resources to safer, user-specific locations:

  • %Program Files%

  • %Windir%

  • %Windir%\System32


When a user process attempts to add a file to a protected folder, UAC redirects the request to the \AppData\Local\VirtualStore\ folder in the user’s profile. For example, if a user named MyUser runs an application that stores a log file at C:\Program Files\MyApps\Logs\Log.txt, the file write attempt will succeed. However, UAC will actually store the file at C:\Users\MyUser\AppData\Local\VirtualStore\Program Files\MyApps\Logs\Log.txt. The application will be able to access the file at C:\Program Files\MyApps\Logs\Log.txt, but the user will need to browse to her profile to access the file directly because virtualization affects only the application process itself. In other words, if the user browses to open the log file from within the application, it will appear to be under %Program Files%. If the users browses to open the log file using an Explorer window, it will be under her profile.

The first time an application makes a change to a virtualized resource, Windows Vista copies the folder or registry key to the location within the user’s profile. Then, the change is made to the user’s copy of that resource.

UAC virtualization is designed to allow already-installed applications to run successfully with standard user privileges, even if they store temporary files or logs in a protected folder. UAC virtualization does not allow users to install applications that make changes to these resources; users will still need to provide administrative credentials to do the installation.

When an executable has a requested execution level manifest, Windows Vista automatically disables UAC virtualization. Therefore, virtualization should never be a factor for applications designed for Windows Vista. Native 64-bit applications are required to be UAC-aware and to write data into the correct locations, and thus are not affected. Virtualization also does not affect applications that administrators run with elevated privileges.

If you plan to run applications that would support virtualization, and you specifically want to prevent UAC from virtualizing requests from the application, you can disable virtualization by using the Application Compatibility Toolkit to mark the application. Setting the NoVirtualization marking makes applications easier to debug (because you don’t have to worry about file and registry requests being redirected) and it reduces the attack surface by making it more difficult for malware to infect an application (because that application’s files would not be moved into the relatively unprotected user profile).

6. UAC and Startup Programs

By default, UAC blocks startup applications located in the Startup folder or identified in the Run registry key that require elevated privileges for both standard and administrative users. It would simply be too annoying to have multiple UAC prompts when logging on, and forcing users to confirm the prompts would require them to blindly elevate processes they did not explicitly launch—a bad security practice.

Instead of a UAC prompt, UAC displays a notification bubble stating that Windows has blocked some startup programs. The user can click the bubble to view and launch the blocked programs within Windows Defender (Windows Defender provides the user interface, but is not itself responsible for blocking startup programs). As an administrator, you should ensure that no startup programs require elevated privileges.

Startup applications launched from the RunOnce registry key or specified in a Group Policy setting are unaffected by this feature; UAC will still prompt the user for administrative credentials. This allows applications that must make changes after restarting the computer to complete installation successfully.

7. Compatibility Problems with UAC

For applications to receive the Certified For Windows Vista logo, the application must be designed to work well for standard users unless the tool is specifically intended for use by administrators. However, many applications were developed prior to Windows Vista and will not work correctly with UAC enabled. Some of the more significant applications that might experience problems with specific features include:

  • Microsoft SQL Server Desktop Engine (MSDE) MSDE is not supported on Windows Vista, but SQL Server 2005 Express is.

  • Microsoft Visual Studio 6.0 and Microsoft Visual Studio 2003 Microsoft currently has no plans to support these versions of Microsoft Visual Studio on Windows Vista.

  • Microsoft Visual Studio 2005 Windows Vista Service Pack 1 must be installed for compatibility with Windows Vista.

Additionally, some older antispyware, antivirus, firewalls, CD/DVD-authoring, disk-defragmentation, and video-editing tools designed for Windows XP will not work with Windows Vista.

Typically, most features of an application will work correctly with UAC enabled, but specific features might fail. You have several ways to work around this:

  • Run the application with administrative credentialsYou can specify that an application always requires administrative credentials.

  • Modify permissions on the computer If an application requires access to a protected resource, you can change the permissions on that resource so that standard users have the necessary privileges. Instructions on how to isolate the protected resources are provided later in this section.

  • Run Windows XP (or an earlier version of Windows) in a virtual machine If the application fails with administrative privileges or you do not want to grant the application administrative privileges to your computer, you can run the application within a virtual machine. Virtual machines provide an operating system within a sandbox environment, allowing you to run applications within Windows XP without requiring a separate computer. You can maximize virtual machines so that they display full-screen, providing a similar experience to running the operating system natively. Virtual machines perform slightly slower than applications that run natively within Windows Vista, however. Virtual PC is a free download from Microsoft, and Windows Vista Enterprise and Windows Vista Ultimate include the right to install Windows XP in a virtual machine without paying additional fees.

  • Disable UAC You can disable UAC to bypass most application-compatibility problems related to the permission changes in Windows Vista. However, this increases the security risks of client computers when running any application, and therefore is not recommended. To disable UAC, read “How to Configure User Account Control” later in this section.

To isolate the protected resources accessed by an application, follow these steps:

On a Windows Vista computer with UAC enabled, download and install the Microsoft Application Verifier from http://www.microsoft.com/downloads/details.aspx?FamilyID=bd02c19c-1250-433c-8c1b-2619bd93b3a2.

On the same computer, download and install the Microsoft Standard User Analyzer from http://www.microsoft.com/downloads/details.aspx?FamilyId=DF59B474-C0B7-4422-8C70-B0D9D3D2F575.

Start the Standard User Analyzer. On the App Info tab, click Browse and then select the application’s executable file.

Click Launch, and respond to any UAC prompts that appear. The Standard User Analyzer will start the application. Use the application, especially any aspects that might require elevated privileges, and then close the application.

Click the View menu and select Detailed Information.

Wait a few moments for the Standard User Analyzer to examine the application log file, as shown in Figure 6. Browse the different tabs to examine any errors. Errors indicate that the application attempted to perform an action that would have failed if it were not run with administrative privileges.

Figure 6. The Standard User Analyzer tool shows exactly which elevated privileges an application requires.

On the File tab and Registry tab (as shown in Figure 7), notice the Work With Virtualization column. If the column is Yes, that particular error will not cause a problem as long as UAC virtualization is enabled. If UAC virtualization is disabled, the error will still occur. If the column is No, it will always be a problem unless the application is run as an administrator.

Figure 7. The Standard User Analyzer tool indicates which problems can be negated by UAC virtualization.
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