A KSY spec for Microsoft CAB files by @computerquip (https://github.com/kaitai-io/kaitai_struct/issues/817#issuecomment-705247470)

microsoft_cab.ksy

meta:
  id: cab
  file-extension: cab
  endian: le
  license: CC0-1.0
doc-ref: http://download.microsoft.com/download/4/d/a/4da14f27-b4ef-4170-a6e6-5b1ef85b1baa/[ms-cab].pdf

seq:
  - id: magic
    contents: "MSCF"
  - id: reserved
    type: u4
  - id: cabinet_size
    type: u4
  - id: cabinet
    type: cabinet
    size: cabinet_size - 12

instances:
  data_reserve_size:
    value: 'cabinet.header.reserve_header.data_reserve_size'
  header_reserve_size:
    value: 'cabinet.header.reserve_header.header_reserve_size'
  folder_reserve_size:
    value: 'cabinet.header.reserve_header.folder_reserve_size'
  prev_cabinet_flag:
    value: '(cabinet.header.flags & 0x0001) > 0'
  next_cabinet_flag:
    value: '(cabinet.header.flags & 0x0002) > 0'
  reserve_present_flag:
    value: '(cabinet.header.flags & 0x0004) > 0'
  folder_count:
    value: 'cabinet.header.folder_count'
  file_count:
    value: 'cabinet.header.file_count'

types:
  cabinet:
    seq:
      - id: header
        type: cf_header
      - id: folders
        type: cf_folder
        repeat: expr
        repeat-expr: _root.folder_count
      - id: files
        type: cf_file
        repeat: expr
        repeat-expr: _root.file_count
      - id: entries
        type: cf_data
        repeat: eos

  cf_header:
    seq:
      - id: reserved1
        type: u4
      - id: files_offset
        type: u4
      - id: reserved2
        type: u4
      - id: version_minor
        type: u1
      - id: version_major
        type: u1
      - id: folder_count
        type: u2le
      - id: file_count
        type: u2
      - id: flags
        type: u2
      - id: set_id
        type: u2
      - id: cabinet_seq
        type: u2
      - id: reserve_header
        type: reserve_header
        if: _root.reserve_present_flag
      - id: prev
        type: cabinet_desc
        if: _root.prev_cabinet_flag
      - id: next
        type: cabinet_desc
        if: _root.next_cabinet_flag

  reserve_header:
    seq:
      - id: header_reserve_size
        type: u2
      - id: folder_reserve_size
        type: u1
      - id: data_reserve_size
        type: u1
      - id: reserve_data
        size: header_reserve_size
        if: header_reserve_size > 0

  cabinet_desc:
    seq:
      - id: cabinet_name
        type: strz
        encoding: ASCII
      - id: disk_name
        type: strz
        encoding: ASCII

  cf_folder:
    seq:
      - id: cab_start_offset
        type: u4
      - id: cf_data_count
        type: u2
      - id: type_compress
        type: u2
        enum: compression_type
      - id: reserve
        size: _root.folder_reserve_size
        if: _root.reserve_present_flag and _root.folder_reserve_size > 0
  
  cf_file:
    seq:
      - id: file_size
        type: u4
      - id: folder_start_offset
        type: u4
      - id: index
        type: u2
      - id: date
        type: u2
      - id: time
        type: u2
      - id: attribs
        type: u2
      - id: name
        type: strz
        encoding: ASCII

  cf_data:
    seq:
      - id: checksum
        type: u4
      - id: data_size
        type: u2
      - id: uncompressed_size
        type: u2
      - id: reserve
        size: _root.data_reserve_size
        if: _root.reserve_present_flag and _root.data_reserve_size > 0
      - id: data
        size: data_size
        if: data_size > 0
      
enums:
  compression_type:
    0x0000: none
    0x0001: mszip
    0x0002: quantum
    0x0003: lzx