Module: FormatParser::ZIPParser::OfficeFormats

Included in:
FormatParser::ZIPParser
Defined in:
lib/parsers/zip_parser/office_formats.rb

Overview

Based on an unscientific sample of 63 documents I could find on my hard drive, all docx/pptx/xlsx files contain, at the minimum, the following files:

[Content_types].xml
_rels/.rels
docProps/core.xml
docPropx/app.xml

Additionally, per file type, they contain the following:

word/document.xml
xl/workbook.xml
ppt/presentation.xml

These are sufficient to say with certainty that a ZIP is in fact an Office document. Also that unscientific sample revealed that I came to dislike MS Office so much as to only have 63 documents on my entire workstation.

We do not perform the actual decoding of the Office documents here, because to read their contents we need to:

  • inflate the compressed part files (potential for deflate bombs)

  • parse the document XML (potential for XML parser exploitation)

which are real threats and require adequate mitigation. For our purposes the token detection of specific filenames should be enough to say with certainty that a document is an Office document, and not just a ZIP.

Constant Summary collapse

OFFICE_MARKER_FILES = 
Set.new([
  '[Content_Types].xml',
  '_rels/.rels',
  'docProps/core.xml',
  'docProps/app.xml',
])

Instance Method Summary collapse

Instance Method Details

#office_document?(filenames_set) ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/parsers/zip_parser/office_formats.rb', line 36

def office_document?(filenames_set)
  OFFICE_MARKER_FILES.subset?(filenames_set)
end

#office_file_format_and_mime_type_from_entry_set(filenames_set) ⇒ Object 



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# File 'lib/parsers/zip_parser/office_formats.rb', line 40

def office_file_format_and_mime_type_from_entry_set(filenames_set)
  if filenames_set.include?('word/document.xml')
    [:docx, 'application/vnd.openxmlformats-officedocument.wordprocessingml.document']
  elsif filenames_set.include?('xl/workbook.xml')
    [:xlsx, 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet']
  elsif filenames_set.include?('ppt/presentation.xml')
    [:pptx, 'application/vnd.openxmlformats-officedocument.presentationml.presentation']
  else
    [:unknown, 'application/zip']
  end
end