Microsoft Cabinet MIME Type

What does application/vnd.ms-cab-compressed mean?

The application/vnd.ms-cab-compressed media type identifies a Microsoft Cabinet archive. A CAB file is a binary container, normally named with the .cab extension, that packages one or more related files and can compress their data.¹²

Microsoft designed the format for Windows software distribution and installation workflows. Windows Installer can keep a cabinet either as a separate file or as a data stream embedded inside an .msi package.³ CAB is still its own archive format, however. A .cab file is not an MSI database, ZIP archive, or executable.

How do you identify a CAB file?

A standard CAB file starts with the ASCII characters MSCF, represented by the bytes 4D 53 43 46. Microsoft's format specification defines this four-byte signature as the first field in the cabinet header and uses it to confirm that the file is a cabinet.²

The opening bytes typically look like this:

4D 53 43 46 00 00 00 00 ...
 M  S  C  F

IANA's 2005 registration says “Magic number(s): none,” while Microsoft's later published format specification explicitly defines the MSCF signature.¹² This page uses the format specification for file identification and notes the discrepancy rather than treating the registration's older field as a description of the current specification.

An extension or four-byte signature is only an initial check. A valid parser must also read the header offsets, folder and file records, data blocks, and checksums. Renaming package.zip to package.cab does not convert it.

How is a Cabinet archive organized?

The CAB format groups file data into folders. In CAB terminology, a folder is a compression unit, not necessarily a directory from the original file system. Every stored file belongs completely to one folder, while a folder can continue across multiple cabinet files in a linked set.²

Microsoft's specification supports uncompressed data, MSZIP, and LZX compression.² Compression can work across file boundaries within a folder, which Microsoft notes can improve the ratio when several related files are packaged together.⁴ The tradeoff is that a program may need to decode a shared stream to recover an individual member.

CAB sets were designed for distribution media with size limits. A large folder may span several numbered cabinets, and each cabinet header can name the logically previous and next files in the set.² If you receive only one part of such a set, extraction may fail even though its MSCF header is valid.

How do you open an application/vnd.ms-cab-compressed file?

Web browsers do not provide a native CAB archive browser. They normally hand the file off as a download. Use an archive utility and inspect the member list before extracting.

Windows includes the expand command. Microsoft documents these forms for listing and extracting cabinet contents:⁵

expand -d package.cab
expand package.cab -f:* output

The destination directory must already exist when extracting multiple files. The -d form lists members without writing them. Microsoft also provides makecab for creating cabinets.⁶

7-Zip is a cross-version Windows option with a graphical file manager and command-line interface. Its official format list includes CAB as an unpack-only format, so it can extract a CAB but does not create one.⁷

Support in Poly

Poly recognizes .cab filenames as application/vnd.ms-cab-compressed and classifies them as archives. You can store, sync, share, version, download, and find the CAB file by name.

Poly treats the payload itself as opaque:

• It does not extract the cabinet or index member filenames and contents.
• It does not generate a thumbnail or provide an in-app archive browser.
• Search can match the CAB file's name and ordinary Poly properties, but not text packed inside it.
• The Poly agent cannot inspect or reason about the archived members.

Download and extract the cabinet with a trusted utility when you need its contents. If you add the extracted files to Poly separately, each file can use the processing pipeline available for its own format.

Is a CAB file the same as an MSI file?

No. A CAB is an archive of file payloads. An MSI is a Windows Installer package whose database describes components, features, installation actions, and source media.

The two formats often work together. Windows Installer may reference a separate cabinet at the root of its source tree, or store a cabinet as a stream inside the MSI package.³ Extracting that cabinet reveals payload files, but it does not reproduce or execute the installation logic held in the MSI database.

Do not change a CAB extension to .msi or .exe. If your goal is only to inspect files, extract them. If the cabinet is part of a trusted driver, update, or application installer, follow the publisher's installation instructions instead of guessing from the extension.

How do you convert CAB to ZIP?

Archive conversion is a two-step operation: extract the CAB members, then create a new ZIP archive from those files. It changes the container, not the formats of the member files.

ZIP is usually a practical choice when recipients need broad archive-tool compatibility. Keep the original CAB when it belongs to an installer or a linked cabinet set. Repacking can discard CAB-specific organization and will not update an MSI or other package that expects the original cabinet name, layout, or checksums.

Security considerations

IANA's registration says the security issues for this media type had not been assessed.¹ That is not a claim that CAB files are safe. A cabinet can carry programs, scripts, drivers, or other active content, and extraction turns those members into ordinary files on disk.

List the archive first, extract it into an isolated directory, and scan the results before opening or running anything. Be especially careful when a cabinet arrived unexpectedly or asks to be installed with elevated privileges. A valid header confirms the container format, not the trustworthiness of its contents.