What is CAB? Windows Cabinet Archive Format Explained

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 .cab file is a Microsoft Cabinet, an archive format built specifically for packaging Windows software for installation. Like ZIP it both bundles and compresses files in a single container, but it is engineered around the needs of installers: reliable extraction, strong compression, and the ability to span an archive across multiple files or media, a feature that mattered when software shipped on several floppy disks or CDs.

Inside, a cabinet is organized into folders, which are compression units rather than directories, each holding one or more files compressed together as a continuous stream. Compressing related files in the same folder lets the algorithm exploit similarity between them for a better ratio, an idea sometimes called solid compression. A header and folder tables describe the layout so the extractor can locate each file.

CAB supports several compression methods, the most capable being LZX, a strong LZ-based algorithm, alongside the lighter MSZIP (a DEFLATE variant) and the option of no compression. LZX in particular gives cabinets a notably good ratio for their era, which is one reason Microsoft used the format so widely for system components.

Because it was designed for installers, CAB integrates tightly with Windows: the operating system can read cabinets natively, and they are the payload inside many .msi installer packages and Windows update bundles. Tools like expand and extract unpack them, and the format supports digital context that installers rely on to place files correctly.

History and Standardization

Microsoft introduced the Cabinet format in the 1990s as the standard way to distribute and install Windows software, replacing looser collections of compressed files. It quickly became a core part of the Windows setup and update machinery.

CAB is a proprietary Microsoft format rather than an open standard, though Microsoft published documentation describing its structure, which allowed third-party and open-source tools to read and create cabinets. That documentation is why utilities outside Windows can extract .cab files today.

The format remains deeply embedded in the Windows ecosystem. It underlies MSI installers, driver packages, and Windows Update, so even though ordinary users rarely handle a .cab directly, the format is quietly involved almost every time Windows software is installed or updated.

CAB Compared to ZIP and MSI

Against ZIP, CAB's advantages are its stronger LZX compression and its solid-compression folders, which let it pack similar files more tightly than ZIP's per-file approach. ZIP is far more universal across platforms, so CAB is chosen where its Windows integration and installer features are the point rather than portability.

CAB is easy to confuse with MSI, but they are different layers. An .msi is a Windows Installer database that describes what to install and how, and it frequently contains or references CAB files that hold the actual compressed payload. So CAB is the packaging of the bytes, while MSI is the installation logic around them.

For everyday file sharing CAB offers little reason to choose it over ZIP or a modern high-ratio archiver, and its cross-platform support, while present, is less seamless. Its enduring value is specific: it is the native, well-integrated container for delivering software and system files inside Windows.

MKV Technical Specifications

CAB vs Other Archive Formats

CAB integrates tightly with Windows setup tooling, while ZIP and 7z are more portable across platforms.

Advantages & Disadvantages

Common Use Cases

CAB is used primarily within the Windows software distribution and installation pipeline.