What is CAB? Windows Cabinet Archive Format Explained
Microsoft's compressed archive format for bundling and installing Windows software components.
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What is CAB?
CAB, short for Cabinet, is an archive file format developed by Microsoft for the Windows platform. It supports lossless data compression and can store embedded digital certificates to verify the integrity of the archived contents.
A cabinet file is identified by the magic bytes 'MSCF' at the start of the file and groups multiple files into folders that are compressed as a unit. It can use either the MSZIP (DEFLATE-based) or the more efficient LZX compression algorithm, and large installations can span multiple cabinet volumes.
How CAB Works
A cabinet groups files into one or more folders, where a folder is a compression unit rather than a directory. Data within a folder is split into roughly 32 KB blocks that are compressed sequentially, which allows the compressor to carry dictionary state across files in the same folder.[2] The header, folder, and file structures are described by Microsoft's published specification.[1]
Compression and Spanning
CAB supports no compression, MSZIP (a DEFLATE-based scheme), and LZX, which generally achieves higher ratios at the cost of more processing.[1] Large software distributions can split a folder across several cabinet files, enabling installers that once spanned multiple floppy disks or CDs.[2]
Usage and Identification
CAB is registered under the media type application/vnd.ms-cab-compressed and underpins many Windows installation technologies, including Setup API and Windows Update packages.[3] Because cross-folder solid compression ties files together, extracting a single file may require decompressing earlier data in the same folder.[2]
MKV Technical Specifications
CAB vs Other Archive Formats
| Feature | CAB | ZIP | 7z |
|---|---|---|---|
| Compression algorithm | LZX / MSZIP[2] | DEFLATE | LZMA / LZMA2 |
| Compression ratio | High | Moderate | Very high |
| Open/proprietary | Microsoft[1] | Open | Open |
| OS support | Mainly Windows | Universal | Cross-platform |
| Best for | Windows installers[3] | General sharing | Max compression |
CAB integrates tightly with Windows setup tooling, while ZIP and 7z are more portable across platforms.
Advantages & Disadvantages
Advantages
Supports the LZX algorithm, which achieves higher compression ratios than the DEFLATE-based MSZIP method.
Cabinets can carry embedded digital certificates, allowing Windows to verify authenticity before installation.
A single logical archive can be split across several .cab files to suit removable media or download chunks.
CAB is handled directly by Windows Setup, Windows Update, and built-in tools such as expand and extract.
Disadvantages
Tooling and ecosystem support are concentrated on Windows, with limited native handling on other platforms.
CAB has largely been superseded by MSI, APPX/MSIX, and modern installer formats for application distribution.
It is rarely chosen for everyday file compression compared with ZIP or 7z.
Common Use Cases
CAB is used primarily within the Windows software distribution and installation pipeline.
Software installation | FileFormer
Setup packages and MSI installers store compressed payloads inside cabinet files.
Device drivers | FileFormer
Windows driver packages and Windows Update deliver components as signed CAB archives.
System updates | FileFormer
Windows servicing uses CAB files to package and deploy operating system updates.
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Try Archive Converter FreeFrequently Asked Questions
How do I open a CAB file on Windows?
Windows can open CAB files directly in File Explorer, and the built-in expand and extract command-line tools can decompress their contents.
What compression does CAB use?
CAB supports MSZIP, which is based on DEFLATE, and LZX, which generally yields smaller files.
Is CAB the same as ZIP?
No. Both are compressed archives, but CAB uses Microsoft's own container format and compression methods and is geared toward Windows software deployment.
Can a CAB file be digitally signed?
Yes. Cabinets can embed digital certificates so that Windows can verify the integrity and origin of the archive.
Is CAB still used today?
Yes, particularly for drivers and Windows updates, though application installers increasingly use MSI or MSIX.