The Windows Update Standalone Installer (WUSA) is a command-line utility integral to the Windows operating system, designed to facilitate the installation and management of standalone update packages, commonly known as .msu files. Introduced with Windows Vista, WUSA has been a consistent feature across subsequent Windows versions, including Windows 7, 8, 10, and 11.
Understanding .msu Files
An .msu file is a Microsoft Update Standalone Package that contains the necessary components for a Windows update. Each .msu file typically includes:
- Windows Update Metadata: Describes the update package contents.
- One or More .cab Files: Each representing an individual update.
- An .xml File: Provides a description of the .msu package, utilized during unattended installations.
- A Properties File: Contains string properties used by WUSA, such as the title of the associated Knowledge Base article.
WUSA operates by extracting the contents of an .msu file to a temporary directory and then performing the following steps:
- Applicability Check: Utilizes the Windows Update metadata to determine if the update is applicable to the system.
- Copying Update Contents: Transfers the relevant update files to the Windows Update sandbox, a protected folder.
- Installation: Invokes the appropriate functions in the Windows Update Agent API to install the update.
- Completion and Status Reporting: After installation, WUSA synchronizes with the Windows Update Agent API to report the installation status.
WUSA offers several command-line switches to control its behavior:
- /quiet: Runs WUSA in quiet mode without user interaction. The system will restart automatically if required.
- /norestart: Prevents WUSA from restarting the computer. This switch is ignored if used without /quiet.
- /uninstall: Uninstalls the specified update.
- /kb:<KB Number>: Specifies the Knowledge Base number of the update to be uninstalled; used in conjunction with /uninstall.
- /extract: Extracts the package contents to a specified folder.
- /warnrestart: When combined with /quiet, prompts before initiating a restart.
- /forcerestart: When combined with /quiet, forces a restart after installation.
- To install an update silently without restarting:
wusa.exe C:\path\to\update.msu /quiet /norestart- To uninstall an update by its KB number:
wusa.exe /uninstall /kb:123456Limitations and Considerations
While WUSA is a powerful tool, it has certain limitations:
- Uninstallation Restrictions: In Windows Vista and Windows Server 2008, WUSA does not support the uninstallation of updates.
- Single Instance Operation: Only one instance of WUSA can run at a time; attempting to run multiple instances concurrently is not supported.
- Applicability Checks: WUSA will not install updates that are not applicable to the system, ensuring system integrity.
It's important to note that certain switches, such as /quiet, have been deprecated in newer Windows versions due to security concerns. Malicious software could potentially exploit these switches to uninstall updates without user intervention. Therefore, it's advisable to use WUSA with caution and ensure that updates are managed securely.
Conclusion
The Windows Update Standalone Installer (WUSA) is an essential utility for managing Windows updates, especially in scenarios requiring manual installation or troubleshooting. By understanding its functionality and command-line options, users and administrators can effectively maintain and update Windows systems.
Source: Microsoft Support Description of the Windows Update Standalone Installer in Windows - Microsoft Support