What Is a MUM File?

By Dan Howard

MUM files contain Windows Update data that keeps your system secure.
i Stockbyte/Stockbyte/Getty Images

When you obtain a Windows security update through Windows Update or manually from the Windows Technet website, several files may be saved on your computer. A MUM file is one such file. It contains information about how the Windows Update application should apply a security patch or software update.


When Microsoft discovers a security vulnerability or issues a software update for the Windows operating system, it issues a patch that fixes the problem. These patches are issued via the Microsoft Windows Update application that comes standard with all versions of Windows. A file with the extension .MUM is an update file that contains information that Windows Update uses to execute the patch on your computer.

Naming Convention

Microsoft employs standard naming conventions for their MUM Windows update files. You can tell which security update a MUM file contains simply by looking at the file name. The first part of the file name describes which package number the file contains. The second part describes the "kb" number, which is the number that Microsoft assigns to its security bulletins. The latter part of the file name shows the operating system version and the release version of the update file. You can find out more about the contents of a MUM update package on your computer by navigating to technet.microsoft.com/en-us/security/bulletin and clicking the "Search by KB Article Number" link.

Windows Update Installation

The most common way to install a MUM update package is through the Windows Update application. Click the "Start" button, then click the "Windows Update" link in your Start menu to initiate the application. Windows Update will automatically detect available updates that have not yet been applied to your computer, including the update contained in your MUM file. Simply click "Install Updates," and wait until Windows Update completes the installation process.

Command Line Installation

MUM files are not designed to be installed directly, either from a command line or by double-clicking on the file. Rather, MUM files contain update data that is accessed by an associated executable MSU update package. If you want to install a MUM package manually, you can install its associated MSU file using the wusa.exe tool in the Windows System32 folder. From a command prompt, type "wusa.exe" followed by the complete path and filename of your MSU update package. This procedure should only be used by IT professionals and other computer experts who want to customize their Windows Update experience.