File extension

.msi

Windows Installer package

.msi files are Windows Installer packages: structured-storage database files that describe how Windows installs, updates, and removes software.

Known meanings

Formats and reference material for .msi

Windows Installer package

Database-based installation package processed by the Windows Installer service

  • windows binaries
  • Microsoft
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Details

Format notes

MSI

An MSI file is an installation package for the Windows Installer service (msiexec.exe). Instead of containing an installer program, an MSI is data: a relational database that tells the service which files, registry entries, shortcuts and services to install, and in what order.

Structure

An MSI is an OLE compound (structured storage) file, so it starts with the bytes D0 CF 11 E0 A1 B1 1A E1, the same signature as legacy Office documents. Inside are tables such as File, Component, Feature, Registry and InstallExecuteSequence, plus summary information and, optionally, embedded cabinet (.cab) archives holding the files to install. Related files include .msp (patches), .mst (transforms that customize a package) and .msm (merge modules).

Tooling

Packages are installed with msiexec (for example msiexec /i package.msi /qn for silent installs), which supports logging and managed deployment. Authoring tools include the WiX Toolset, Visual Studio-based setup projects and commercial products. Installations are transactional and can be rolled back on failure.

Preservation And Security Notes

MSI packages can run custom actions with elevated privileges, so verify the Authenticode signature and the source before installing. Because an MSI is a database, its contents can be inspected without installing it, which helps with auditing and archiving.

Further Reading

  • Windows Installer portal: https://learn.microsoft.com/en-us/windows/win32/msi/windows-installer-portal
  • Overview of Windows Installer: https://learn.microsoft.com/en-us/windows/win32/msi/overview-of-windows-installer
  • Windows Installer file extensions: https://learn.microsoft.com/en-us/windows/win32/msi/windows-installer-file-extensions