Didier Stevens

Monday 16 December 2019

Analyzing .DWG Files With Embedded VBA Macros

Filed under: maldoc,Malware — Didier Stevens @ 0:00

AutoCAD’s drawing files (.dwg) can contain VBA macros. The .dwg format is a proprietary file format. There is some documentation, for example here.

When VBA macros are stored inside a .dwg file, an OLE file is embedded inside the .dwg file. There’s a quick-and-dirty way to find this embedded file inside the .dwg file: search for magic sequence D0CF11E0.

My tool cut-bytes.py can be used to search for the first occurrence of byte sequence D0CF11E0 and extract all bytes starting from this sequence until the end of the .dwg file. This can be done with cut-expression [D0CF11E0]: and pipe the result into oledump.py, like this:

Next, oledump can be used to conduct the analysis as usual, for example by extracting the VBA macro source code:

There is also a more structured approach to locate the embedded OLE file inside a .dwg file. When one looks at a .dwg file with a hexadecimal editor, the following can be seen:

First there is a magic sequence identifying this as a .dwg file: AC1032. This sequence varies with the file format version, but since many, many years, it starts with AC10. You can find more details regarding this magic sequence here and here.

At position 0x24 (36 decimal), there is a 32-bit little-endian integer. This is a pointer to the embedded OLE file (this pointer is NULL when no OLE file with VBA macros is embedded).

In our example, this pointer value is 0x00008080. And here is what can be found at this position inside the .dwg file:

First there is a 16-byte long header. At position 8 inside this header, there is a 32-bit little-endian integer that represents the length of the embedded file. 0x00001C00 in our example. And after the header one can find the embedded OLE file (notice magic sequence D0CF11E0).

This information can then be used to extract the OLE file from the .dwg like, like this:

Achieving exactly he same result as the quick-and-dirty method. The reason we don’t have to figure out the length of embedded OLE the file using the quick-and-dirty method, is that oledump ignores all bytes appended to an OLE file.

I will adapt my oledump.py tool to extract macros directly from .dwg files, without the need of a tool like cut-bytes.py, but I will probably implement something like the quick-and-dirty method, as this method would potentially work for other file formats with embedded OLE files, not only .dwg files.

 

Monday 9 December 2019

Update: oledump.py Version 0.0.43

Filed under: My Software,Update — Didier Stevens @ 0:00

This new version of oledump.py adds support for Python 3. Several plugins and decoders were also updated for Python 3.

There’s a new option to include storages in the overview: –storages.

And option –decompress now does also VBA decompression (it was zlib only). This helps to decompress the dir stream of documents with VBA macros:

And I added type 1009 to plugin_msg.py: Compressed RTF.

oledump_V0_0_43.zip (https)
MD5: F98A06CED73C4FC2CA153B7E751746B5
SHA256: 4FE1DBAB822CEC2489328CE3D4D272400F23F1FAD266C9D89B49D9F83F3AA27F

Sunday 8 December 2019

Update: numbers-to-string.py Version 0.0.9

Filed under: My Software,Update — Didier Stevens @ 19:34

This is just a bugfix version (Python 3).

numbers-to-string_v0_0_9.zip (https)
MD5: C5629F102FCF58E5CFF24472D35AFF22
SHA256: 5B1CA43EDFD7BA66CF44FB552BD7882AEB13A8765017F9F865071E187410EE63

Overview of Content Published in November

Filed under: Announcement — Didier Stevens @ 9:36

Here is an overview of content I published in November:

Blog posts:

SANS ISC Diary entries:

Monday 18 November 2019

Update: tcp-honeypot.py Version 0.0.7

Filed under: My Software,Networking,Update — Didier Stevens @ 0:00

This new version of tcp-honeypot.py, a simple TCP honeypot and listener, brings TCP_ECHO and option -f as new features.

TCP_ECHO can be used to send back any incoming data (echo). Like this:

dListeners = {4444: {THP_LOOP: 10,THP_ECHO: None,},}

TCP_ECHO also takes a function, which’s goal is to transform the incoming data and return it. Here is an example with a lambda function that converts all lowercase letters to uppercase:

dListeners = {4444: {THP_LOOP: 10,THP_ECHO: lambda x: x.upper(),},}

If persistence is required across function calls, a custom class can also be provide. This class has to implement a method with name Process (input: incoming data, output: transformed data). Consult the man page (option -m) for more details.

And option -f (format) can be used to change the output format of data.
Possible values are: repr, x, X, a, A, b, B
The default value (repr) output’s data on a single line using Python’s repr function.
a is an ASCII/HEX dump over several lines, A is an ASCII/HEX dump too, but with duplicate lines removed.
x is an HEX dump over several lines, X is an HEX dump without whitespace.
b is a BASE64 dump over several lines, B is a BASE64 without whitespace.

 

 

Tuesday 12 November 2019

Steganography and Malware

Filed under: Malware,My Software — Didier Stevens @ 0:00

I was reading about malware using WAV files and steganography to download payloads without triggering detection systems.

For example, here is a WAV file with a hidden, embedded PE file. The PE file is encoded in the least significant bit of 16-bit integers that encode PCM sound.

I was wondering how I could extract this embedded file with my tools. There was no easy solution, because many of my tools operate on byte streams, but here I have to operate on a bit stream. So I made an update to my format-bytes.py tool.

Using my tool file-magic.py, I get confirmation that this is a sound file (.WAV) with 16-bit PCM data.

And here is an ASCII/HEX dump of the beginning of the file made with cut-bytes.py:

The data chunk starts with magic sequence ‘data’ (in yellow), followed by the size of the data chunk (in green), and then the data itself: 16-bit, little-endian signed integers (in red).

To extract the least significant bit of each 16-bit, little-endian signed integer and assemble them into bytes, I use the latest version of format-bytes.py.

This is the command that I use:

format-bytes.py -a -f “bitstream=f:<H,b:0,j:<” #c#[‘data’]+8: DB043392816146BBE6E9F3FE669459FEA52A82A77A033C86FD5BC2F4569839C9.wav.vir

With option -f, I specify a bitstream format.

f:<H means that the format of the data is little-endian (<), unsigned 16-bit integers (H). I could also specify a signed 16-bit integer (h), but this doesn’t matter here, as I’m not going to use the sign of the integers.

b:0 means that I extract the least-significant bit (position 0) of each 16-bit integer.

j:< means that I assemble (join) these bits into bytes from least significant to most significant (<).

The data starts 8 bytes into the data chunk, e.g. 8 bytes after magic sequence ‘data’. I define this with cut-expression #c#[‘data’]+8:.

When I run this command, and perform an ASCII dump, I get this output for the beginning of the stream:

I can indeed see an executable (MZ), but it is preceded by 4 bytes. These 4 bytes are the length of the embedded file. As described in the article, the length is big-endian encoded. Hence I use a similar command to extract the length, but with j:>, as can be seen here:

The length is 733696 bytes, and this matches the IOCs from the article.

Then I use my tool pecheck.py to search for PE files inside the byte stream (-l P), like this:

MD5 7cb0e1e2cf4a9bf450a350a759490057 is indeed the hash of the malicious DLL encoded in this WAV file.

 

 

 

 

 

Saturday 9 November 2019

Update: format-bytes.py Version 0.0.10

Filed under: My Software,Update — Didier Stevens @ 0:00

This new version of format-bytes.py, a tool to parse binary data, comes with support for bit streams.

This can help, for example, with decoding steganographic data, like a PE file hidden in a .WAV file.

More about this in an upcoming blog post.


format-bytes_V0_0_10.zip (https)
MD5: 3349E2F8C84AE644C0AEFDA4410297C5
SHA256: F75C3A353E42D847264702B1F316A65657E6375EF979B8EF21B282D4676BE4C3

Sunday 3 November 2019

Update: numbers-to-string.py Version 0.0.10

Filed under: My Software,Update — Didier Stevens @ 0:00

numbers-to-string.py is a tool to help with deobfuscation: it transforms numbers found in its input into strings.

This new version adds option -b to produce binary output.

numbers-to-string_v0_0_8.zip (https)
MD5: 69179F5EE01F8E0102F40B768E80A82E
SHA256: 535518780E9F4102320C81EF799CF1AD483C51450690A2E1FA9F2CA61B7A8A88

Saturday 2 November 2019

Update: cut-bytes.py Version 0.0.10

Filed under: My Software,Update — Didier Stevens @ 0:00

This new version of cut-bytes.py, a tool to select a byte sequence from its input, has bug fixes (including Python 3 fixes) and 2 new options: -p –prefix and -s –suffix.

With these options, arbitrary data can be prefixed or appended to the input.

cut-bytes_V0_0_10.zip (https)
MD5: C14F60F9843F4C2A40A05A52CBE16AB8
SHA256: AD3ADBF30B09DB77B17FEF62C40CDC138516FD24B077201D126D259D1953792B

Friday 1 November 2019

Overview of Content Published in October

Filed under: Announcement — Didier Stevens @ 0:00

Here is an overview of content I published in October:

Blog posts:

SANS ISC Diary entries:

NVISO blog posts:

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