.CAB

Microsoft Cabinet

Introduction

Cabinet is Microsoft’s archive format for installing software. It has been used since the mid-1990s for Windows setup files, drivers, updates and Office installers, and Windows Installer packages often carry their files in cabinets. A set of cabinets can be chained so one install spans several files or disks. Windows opens .cab files like folders, and its expand and makecab commands unpack and build them. Microsoft publishes the format as [MS-CAB].

Structure

Files are grouped into folders, each compressed as one stream, so small files in the same folder compress well together. Numbers are little-endian.

  1. 0Header MSCF, cabinet size, offset of the first file entry, version 1.3, folder and file counts, flags, set ID and number in the set36 bytes or more
  2. …Folder entries offset of the first data block, block count, compression type8 bytes each
  3. …File entries size, offset in the uncompressed folder, folder index, date, time, attributes, name16 bytes + name each
  4. …Data blocks checksum, compressed and uncompressed size, dataup to 32 KB uncompressed each
A cabinet: header, folders, files, then the compressed data.

The compression type is none, MSZIP (DEFLATE in 32 KB blocks), Quantum or LZX. Header flags say whether the cabinet continues from a previous one or into a next one, and whether optional reserved areas are present for signing.

Links

Software

Developers

  • libmspack, C library for cabinets and other Microsoft compression formats
  • libarchive, C library that reads cabinets

Reference

  • [MS-CAB]: Cabinet File Format, Microsoft Open Specifications