Hidden malicious modules in MS VBA (Visual Basic for Applications)

["Thegrideon Software" <info () thegrideon com>]

Credits:

Maxim Tomashevich / Thegrideon Software

Website:

https://www.thegrideon.com/

Details:

https://www.thegrideon.com/vba-internals.html

 

 

Vendor:

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Microsoft

 

 

Product:

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Visual Basic for Applications (VBA) 6.5 - 7.1 (x32 / x64)

 

 

Vulnerability Details:

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VBA library loads modules for execution from all streams inside "VBA" OLE
folder (based on "dir" stream records), but only the modules present in
"PROJECT" stream are listed and shown in VBA Project Explorer.

"PROJECT" stream modification can be used to hide any module and this
creates easy to exploit security vulnerability:

1) Malicious code of any length and complexity can be hidden without any
security warnings or hints from VBA IDE whatsoever.

2) A single call to a hidden module is all that required to execute
malicious code. This call can be masked by number of tricks (name visually
similar to a standard function for example: latin letter 'O' (oh) replaced
with '0' (zero) or 'o' with Cyrillic or Greek 'o', function name misspelled,
etc.)

Function StrCоmp(a, b) -or- STRC0MP(a, b) -or simply- StrCmp(a, b)

Or Sub auto_open() can be used for self-containing and a bit more dangerous
malicious module if the host file is useful enough to be opened twice.

3) An absence of any notifications about hidden modules presence and an
ability to see all other listed modules together create a false sense of
security and provoke to enable the code that looks harmlessly.

Sub Function1() ...

LResult = StrCоmp ("vba", "vba") -or- LResult = STRC0MP("vba", "vba") -or-
LResult = StrCmp("vba", "vba")

... End Sub

4) This approach can be used to "weaponize" popular VBA scripts in all
VBA-enabled applications: AutoCAD, Excel, CorelDRAW, PowerPoint, Word, some
PLC systems, etc...

 

 

Proof of Concept:

---------------------

You can download MS Excel example files (with hidden and visible code
blocks) from:

https://www.thegrideon.com/vbaint/vba_hidden_module.zip

or create one with the following basic procedure:

1) Create new VBA-enabled ".xslm" Spreadsheet and add 2 modules "Module1",
"Module2".

(at least one visible module [with or without code] is required to keep VBA
library from removing VBA Project on Save)

2) Change file extension from ".xslm" to ".zip", open the file and extract
vbaProject.bin from "xl" folder.

3) With any OLE editor remove line "Module=Module1" from "PROJECT" stream.

Hex editor can be used as well to replace "Module=Module1" with several
empty lines (0xD 0xA).

4) Put modified vbaProject.bin back into archive and change the extension
back to ".xslm".

5) Open the file in MS Excel to verify "Module1" disappearance.

 

 

Severity Level:

---------------------

Low

 

 

Disclaimer:

---------------------

Permission is hereby granted for the redistribution of this text, provided
that it is not altered except by reformatting, and that due credit is given.
Permission is explicitly given for insertion in vulnerability databases and
similar, provided that due credit is given to the author.The author is not
responsible for any misuse of the information contained herein and prohibits
any malicious use of all security related information or exploits by the
author or elsewhere.


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