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Update: oledump.py Version 0.0.50

This new version brings updates to plugin plugin_biff.py.

This plugin can now produce a CSV list of cell values and formulas (option -c) or a JSON file of values and formulas (option -j).

Cell references are in RC format (row-column), but can also be produced in letters-numbers format (LN, option -r LN).

CSV or JSON output can be piped into my ad-hoc decoding programs.


oledump_V0_0_50.zip (https)
MD5: 30EB6A0E0924E72350B268ADDE4E4EC7
SHA256: 870167AE5576B169EB52572788D04F1FFCEC5C8AFDEBCC59FE3B8B01CBDE6CD9

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Analyzing Malformed ZIP Files

With version 0.0.16 (we are now at version 0.0.18), I updated my zipdump.py tool to handle (deliberately) malformed ZIP files. My zipdump tool uses Python’s ZIP module to analyze ZIP files.

Now, zipdump has a an option (-f) to scan arbitrary binary files for ZIP records.

I will show here how this feature can be used, by analyzing a sample Xavier Mertens wrote a diary entry about. This sample is a Word document with macros, an OOXML (Office Open XML format) file (.docm). It is malformed, because 1) there’s an extra byte at the beginning and 2) there’s a byte missing at the end.

When you use my zipdump tool to look at the file, you get an error:

Using option -f l (list), we can find all PKZIP records inside arbitrary, binary files:

When using option -f with value l, a listing will be created of all PKZIP records found in the file, plus extra data. Some of these entries in this report will have an index, that can be used to select the entry.

In this example, 2 entries can be selected:

p: extra bytes at the beginning of the file (prefix)

1: an end-of-central-directory record (PK0506 end)

Using option -f p, we can select the prefix (extra data at the beginning of the file) for further analysis:

And from this hex/ascii dump, we learn that there is one extra byte at the beginning of the ZIP file, and that it is a newline characters (0x0A).

Using option -f 1, we can select the EOCD record to analyze the ZIP file:

As this generates an error, we need to take a closer look at the EOCD record by adding option -i (info):

With this info, we understand that the missing byte makes that the comment length field is one byte short, and this causes the error seen in previous image.

ZIP files can contain comments (for the ZIP container, and also for individual files): these are stored at the end of the PKZIP records, preceded by a 2-byte long, little-endian integer. This integer is the length of the comment. If there is no comment, this integer is zero (0x00).

Hence, the byte we are missing here is a NULL (0x00) byte. We can append a NULL byte to the sample, and then we should be able to analyze the ZIP file. In stead of modifying the sample, I use my tool cut-bytes.py to add a single NULL byte to the file (suffix option: -s #h#00) and then pipe this into zipdump:

File 5 (vbaProject.bin) contains the VBA macros, and can be piped into oledump.py:

I also created a video:

zipdump_v0_0_18.zip (https)
MD5: 34DC469E8CD4E5D3E9520517DEFED888
SHA256: 270B26217755D7ECBCB6D642FBB349856FAA1AE668DB37D8D106B37D062FADBB

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CLSIDs in OLE Files

Directory entries in “OLE” files (Compound File Binary Format) have a GUID field. Like this “Root Entry” inside a binary Word document file (Doc1.doc):

The GUID value found in this directory entry is: 00020906-0000-0000-C000-000000000046 (the endianness of GUIDs is mixed-endian: it’s a mix of little-endian and big-endian).

This GUID is a COM class id (CLSID) for Word.

You can display the CLSID with oledump.py using option -E to display extra information. Use parameter %CLSID% to display the CLSID, like this:

No class IDs were displayed in this output, and that’s because all the CLSID fields in the directory entries of these streams are zero (16 0x00 bytes). Most of the time, streams in Office documents have no CLSID. You’re more likely to find CLSIDs inside the directory entries of storages. To include storages in oledump’s output, use option –storages like this:

Starting with version 0.0.46, oledump.py will also display the Root Entry. And as can be seen in the above output, the Root Entry of this .doc file has a CLSID.

Philippe Lagadec, the developer of olefile and oletools, maintains a list of CLSIDs relevant to Office documents.

When oletools is installed, oledump.py looks up CLSIDs in this list when you use parameter %CLSIDDESC% (CLSID description). Here is the same command as before, but with parameter %CLSIDDESC%:

This result shows that 00020906-0000-0000-C000-000000000046 is COM Object “Microsoft Word 97-2003 Document (Word.Document.8)”.

 

Class IDs can also be found inside some streams, and that’s why I developed a new oledump.py plugin: plugin_clsid.py.

This plugin searches for CLSIDs (defined in oletools) inside streams. Like in this malicious document:

With the class IDs found in this stream, one can quickly conclude that this must be an exploit for the URL moniker.

And here is the Root Entry CLSID for this document: