Application modernisation
CIM
Composite Image File System
CIM, the Composite Image File System, is the container image format that sits underneath an installed MSIX package. It is the specific mechanism that makes MSIX's fast, low-footprint install and mount behaviour possible.
Why CIM matters in a Microsoft estate
CIM matters during Windows 11, Intune, Azure Virtual Desktop, and Cloud PC programmes because application blockers can delay the whole rollout. The practical question is whether the term helps capture, package, sign, deploy, or troubleshoot an app with less rework.
How CIM shows up in practice
Rather than copying every file in a package onto disk during installation the way a traditional installer does, MSIX packages are stored as a CIM image, a single-instance, content-addressed, read-only image format. This image is mounted directly through the cimfs.sys filesystem driver built into Windows, so the application's files appear on disk without a full file-by-file copy operation having to complete first. This is what allows an MSIX install to complete in a fraction of the time a comparable traditional installer takes. It is also the underlying technology behind CimFS-based App Attach on Azure Virtual Desktop and Windows 365, where a CIM image containing an application is attached to a user's session at logon rather than the application being pre-installed into every session host image.
Single-instancing at the CIM layer means that identical file content across packages, or across multiple versions of the same package, can be stored once rather than duplicated. This keeps the storage footprint of an application library smaller than the equivalent set of full installations would be, a genuinely material consideration for organisations running App Attach at scale across large AVD or Windows 365 estates where storage and mount performance both affect user logon time. CIM has effectively superseded VHDX as the preferred container for newer App Attach deployments, because it mounts faster and avoids the fixed or dynamic disk overhead that a virtual hard disk format carries.
VHDX-based App Attach remains in active use and is still fully supported, though, particularly in estates that adopted App Attach before CimFS-based delivery matured or where tooling has not yet been updated to the newer format. For anyone building or troubleshooting an application streaming or attach pipeline, the practical implication is that CIM is not something packagers interact with directly the way they interact with an MSIX manifest or a certificate. It operates transparently underneath the package format, but its presence explains observable behaviour: why an MSIX application can appear to be available almost immediately after attach, why disk usage for a library of similar packages is lower than expected, and why troubleshooting a slow or failed attach sometimes needs to consider the CIM mount layer itself rather than only the package or the application inside it.