This tool was released during our BlackHat Asia talk (March 28, 2019). A video recording of this talk is available at https://www.youtube.com/watch?v=9ULzZA70Dzg.

Evil Clippy

A cross-platform assistant for creating malicious MS Office documents. Can hide VBA macros, stomp VBA code (via P-Code) and confuse macro analysis tools. Runs on Linux, OSX and Windows.

If you're new to this tool, you might want to start by reading our blog post on Evil Clippy: https://outflank.nl/blog/2019/05/05/evil-clippy-ms-office-maldoc-assistant/

This project should be used for authorized testing or educational purposes only.

Current features

• Hide VBA macros from the GUI editor
• VBA stomping (P-code abuse)
• Fool analyst tools
• Serve VBA stomped templates via HTTP
• Set/Remove VBA Project Locked/Unviewable Protection

If you have no idea what all of this is, check out the following resources first:

• Our MS Office Magic Show presentation at Derbycon 2018
• VBA stomping resources by the Walmart security team
• Pcodedmp by Dr. Bontchev

How effective is this?

At the time of writing, this tool is capable of getting a default Cobalt Strike macro to bypass most major antivirus products and various maldoc analysis tools (by using VBA stomping in combination with random module names).

Technology

Evil Clippy uses the OpenMCDF library to manipulate MS Office Compound File Binary Format (CFBF) files, and hereto abuses MS-OVBA specifications and features. It reuses code from Kavod.VBA.Compression to implement the compression algorithm that is used in dir and module streams (see MS-OVBA for relevant specifications).

Evil Clippy compiles perfectly fine with the Mono C# compiler and has been tested on Linux, OSX and Windows.

Compilation

We do not provide a binary release for EvilClippy. Please compile executables yourself:

OSX and Linux

Make sure you have Mono installed. Then execute the following command from the command line:

mcs /reference:OpenMcdf.dll,System.IO.Compression.FileSystem.dll /out:EvilClippy.exe *.cs

Now run Evil Clippy from the command line:

mono EvilClippy.exe -h

Windows

Make sure you have Visual Studio installed. Then execute the following command from a Visual Studio developer command prompt:

csc /reference:OpenMcdf.dll,System.IO.Compression.FileSystem.dll /out:EvilClippy.exe *.cs

Now run Evil Clippy from the command line:

EvilClippy.exe -h

Usage examples

Print help

EvilClippy.exe -h

Hide/Unhide macros from GUI

Hide all macro modules (except the default "ThisDocument" module) from the VBA GUI editor. This is achieved by removing module lines from the project stream [MS-OVBA 2.3.1].

EvilClippy.exe -g macrofile.doc

Undo the changes done by the hide option (-g) so that we can debug the macro in the VBA IDE.

EvilClippy.exe -gg macrofile.doc

Stomp VBA (abuse P-code)

Put fake VBA code from text file fakecode.vba in all modules, while leaving P-code intact. This abuses an undocumented feature of module streams [MS-OVBA 2.3.4.3]. Note that the VBA project version must match the host program in order for the P-code to be executed (see next example for version matching).

EvilClippy.exe -s fakecode.vba macrofile.doc

Note: VBA Stomping does not work for files saved in the Excel 97-2003 Workbook (.xls) format

Set target Office version for VBA stomping

Same as the above, but now explicitly targeting Word 2016 on x86. This means that Word 2016 on x86 will execute the P-code, while other versions of Word wil execute the code from fakecode.vba instead. Achieved by setting the appropriate version bytes in the _VBA_PROJECT stream [MS-OVBA 2.3.4.1].

EvilClippy.exe -s fakecode.vba -t 2016x86 macrofile.doc

Set/reset random module names (fool analyst tools)

Set random ASCII module names in the dir stream [MS-OVBA 2.3.4.2]. This abuses ambiguity in the MODULESTREAMNAME records [MS-OVBA 2.3.4.2.3.2.3] - most analyst tools use the ASCII module names specified here, while MS Office used the Unicode variant. By setting a random ASCII module name most P-code and VBA analysis tools crash, while the actual P-code and VBA still runs fine in Word and Excel.

EvilClippy.exe -r macrofile.doc

Note: this is known to be effective in tricking pcodedmp and VirusTotal

Set ASCII module names in the dir stream to match their Unicode counterparts. This reverses the changes made using the (-r) optoin of EvilClippy

EvilClippy.exe -rr macrofile.doc

Serve a VBA stomped template via HTTP

Service macrofile.dot via HTTP port 8080 after performing VBA stomping. If this file is retrieved, it automatically matches the target's Office version (using its HTTP headers and then setting the _VBA_PROJECT bytes accordingly).

EvilClippy.exe -s fakecode.vba -w 8080 macrofile.dot

Note: The file you are serving must be a template (.dot instead of .doc). You can set a template via a URL (.dot extension is not required!) from the developer toolbar in Word. Also, fakecode.vba must have a VB_Base attribute set for a macro from a template (this means that your fakecode.vba must start with a line such as Attribute VB_Base = "0{00020906-0000-0000-C000-000000000046}").

Set/Remove VBA Project Locked/Unviewable Protection

To set the Locked/Unviewable attributes use the '-u' option:

EvilClippy.exe -u macrofile.doc

To remove the Locked/Unviewable attributes use the '-uu' option:

EvilClippy.exe -uu macrofile.doc

Note: You can remove the Locked/Unviewable attributes on files that were not locked with EvilClippy as well.

Limitations

Developed for Microsoft Word and Excel document manipulation.

As noted above, VBA stomping is not effective against Excel 97-2003 Workbook (.xls) format.

Authors

Stan Hegt (@StanHacked) / Outflank

With significant contributions by Carrie Roberts (@OrOneEqualsOne / Walmart).

Special thanks to Nick Landers (@monoxgas / Silent Break Security) for pointing me towards OpenMCDF.