What is the MUM file format?
A .mum file (Windows Update Manifest) is an XML document that describes a Windows update package to the Component Based Servicing (CBS) engine, introduced with Windows Vista in 2006. It tells the Windows servicing stack exactly which files to install, what registry keys to configure, which access control lists to apply, and how the package relates to other installed components.
The <assembly> root element contains assemblyIdentity (package name, version, architecture, and language), dependency declarations, file payload entries with embedded Base64-encoded SHA-1 and SHA-256 hashes for integrity verification, registry modifications, and NTFS ACL assignments. A companion .cat security catalog file provides the digital signature that authenticates the entire package.
.mum files appear in two contexts: inside a CAB or .msu archive downloaded during Windows Update, and permanently in %windir%\Servicing\Packages\ after installation - one file per installed update. On a well-patched Windows system this folder can hold thousands of manifests.
The files are plain UTF-8 (occasionally UTF-16 LE) XML and open in any text editor, though they are not meant for manual editing. A "MUM Missing" entry in CBS.log indicates update store corruption; the standard repair is DISM /Online /Cleanup-Image /RestoreHealth followed by sfc /scannow.
Security & safety
RISK: MEDIUMLegitimate MUM files are internal Windows system files and are safe. However, malicious installers can bundle fake .mum files to manipulate Windows servicing. Never run a DISM /Add-Package command with an untrusted .mum file. If a third-party tool asks you to copy .mum files into %windir%\Servicing\Packages\ manually, treat it with extreme caution. Corrupted MUM files are a common symptom of Windows Update failure - use DISM to repair, not manual editing.
Format details
in a nutshellPrograms that open MUM files
Windows5 apps
Microsoft Windows Update
Built-in Windows Update reads MUM files automatically - no user action required. 
Microsoft Windows Vista
Paid MUM files are internal system files; Windows uses them automatically during updates. 
Microsoft Windows 7
Paid MUM files are internal system files consumed automatically by Windows Update. 
Microsoft XML Notepad
Open-source File → Open → select .mum file. Displays XML tree with formatting. Technical details
deep spec| XML root element | `<assembly>` containing `assemblyIdentity`, `dependency`, `file`, and `registryKey` nodes |
| File encoding | UTF-8 (most common) or UTF-16 LE with BOM |
| File signature | `<?xml` at byte offset 0; no MUM-specific magic - identified by content and file path |
| Integrity mechanism | SHA-1 and SHA-256 hashes (Base64-encoded) per payload file, embedded in XML |
| Authentication | Companion `.cat` security catalog file provides digital signature for the package |
| Install location | `%windir%\Servicing\Packages\` - one `.mum` per installed Windows update |
| Distribution container | Shipped inside `.cab` or `.msu` archive as part of the Windows Update download |
| Introduced with | Component Based Servicing (CBS) architecture, Windows Vista (November 2006) |
| OS coverage | Windows Vista through Windows 11; Windows Server 2008 through current |
| Corruption repair | `DISM /Online /Cleanup-Image /RestoreHealth` then `sfc /scannow` from an elevated prompt |
| Error symptom | "MUM Missing" in `CBS.log`; `0xc0000034` boot error when a critical manifest is absent |
| Typical file size | 1 KB - 500 KB depending on package scope |
| Editing | Not intended for manual editing; consumed by the CBS (`TrustedInstaller`) service |
| Transactional safety | CBS uses transaction log files to ensure atomic install and uninstall of update packages |
| Released | 2006 (Windows Vista, Component Based Servicing architecture) |
| Latest version | current (used in Windows 11 2024 H2); format has not changed structurally since Vista |
MUM conversions
Community Q&A
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