SANS ISC: Handling Special PDF Compression Methods

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SANS ISC InfoSec Forums

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.