Microsoft Compound File

Revision as of 14:20, 19 November 2017

Microsoft Compound File is a complex container format used by some versions of Microsoft Office, and other Microsoft applications. It has features similar to those of a filesystem format.

It is also known as Compound File Binary File Format (CFBF or CFB), Microsoft Compound Document File Format, OLE Compound Document Format, OLE2 Compound Document Format, Composite Document File, etc.

The format was not publicly documented by Microsoft until 2008.

It is (or was?) unofficially known as LAOLA File Format.

Identification

Files begin with signature bytes D0 CF 11 E0 A1 B1 1A E1.

Identifying the specific document type can be difficult. This is one of the few formats for which the file command resorts to a hard-coded identification algorithm (see readcdf.c).

Some, but not all, document types can be identified by the CLSID field in the "root storage" directory entry. This field is usually located at file offset 512×(1 + {the 32-bit integer at offset 48}) + 80.

Some files have a stream named "<U+0005>SummaryInformation" containing metadata, which may include information about the creating application.

Root storage object CLSIDs

The table below lists some of the root storage object CLSIDs that have been observed in this type of file. Use this information at your own risk, as these identifiers can be unreliable.

Microsoft's documentation says this about the CLSID field:

This field contains an object class GUID. [...] If not [all zeroes], the object class GUID can be used as a parameter to start applications.

Although every storage object (think subdirectory) can have a CLSID, this table is only concerned with root storage objects.

Note that the CLSIDs are stored as GUIDs in little-endian binary format, so they have a strange byte order.

Related formats

See Category:Microsoft Compound File.

Specifications

• MSDN: Compound File Binary File Format → [MS-CFB] PDF
• OpenOffice.org's documentation

Programs, libraries, and utilities

• Structured Storage Viewer
• libolecf
• OleFileIO_PL - a Python module to read MS OLE2 files
• oledump.py beta
• officeparser
• python-oletools - python tools to analyze OLE files
• OpenMCDF
• 7-Zip

Links

• Wikipedia article
• Forensics Wiki article
• Joel on Software: Why are the Microsoft Office file formats so complicated?
• Some (older) reverse engineered information here and a Perl module here
• Malicious Office macros are not dead
• MS Office 97-2003 legacy/binary formats security - article with lots of resources on MS Office formats, including analysis techniques, tools and parsing libraries
• MSDN: Providing Summary Information

Editors' notes

TODO: Explain the relationship between Compound File format and the format/technology called COM Structured Storage (or OLE Structured Storage).