Threat Level: green Handler on Duty: Didier Stevens

SANS ISC: Maldoc: Excel 4 Macros in OOXML Format SANS ISC InfoSec Forums

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Maldoc: Excel 4 Macros in OOXML Format

I've mentioned Excel 4 macros before, a scripting technology that predates VBA.

In that diary entry, I handle .xls files (ole files). Excel 4 macros can also be stored in Office Open XML format files: .xlsm files.

If we take a look at an .xlsm file with Excel 4 macros with oledump.py, we'll get this output:

There is no ole file (vbaProject.bin) file inside an Excel 4 macro-only file.

We need to take a look with zipdump.py:

The presence of folder macrosheets tells us that there are Excel 4 macro sheets inside this file.

We can look at the content of the XML file:

And pretty-print it with xmldump.py:

Now it's easier to spot the formulas: EXEC("calc.exe") and HALT()

And the Auto_Open can be found in the worksheet XML file:

It's possible to have both macro types inside the same file: Excel 4 and VBA macros. I'll cover that in an upcoming diary entry.

 

Didier Stevens
Senior handler
Microsoft MVP
blog.DidierStevens.com DidierStevensLabs.com

DidierStevens

458 Posts
ISC Handler
Feb 24th 2020

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