Specifications

Obfuscation and (non-)detection of malicious PDF files

More than two months ago I talked at Rooted CON (Madrid) about some techniques to obfuscate and hide malicious PDF files. I gave the same speech at CARO 2011 (Prague) some days ago with updated slides and a demo of peepdf.
 

 
The idea is that it's possible to use some malformations in the documents, like those commented by Julia Wolf, and the PDF specification itself in order to keep the files hidden from Antivirus engines and parsers. Bad guys can effectively use it to create an undetectable exploit and use it as an attacking vector. Some of the techniques are the following:

 

peepdf - PDF Analysis Tool


 

 


What is this?


peepdf is a Python tool to explore PDF files in order to find out if the file can be harmful or not. The aim of this tool is to provide all the necessary components that a security researcher could need in a PDF analysis without using 3 or 4 tools to make all the tasks. With peepdf it's possible to see all the objects in the document showing the suspicious elements, supports the most used filters and encodings, it can parse different versions of a file, object streams and encrypted files. With the installation of PyV8 and Pylibemu it provides Javascript and shellcode analysis wrappers too. Apart of this it is able to create new PDF files, modify existent ones and obfuscate them.

 


Usage


 

Security PDF-related links in 2010: analyses and tools

After one year full of security issues related to the Portable Document Format I've made a little compilation of useful links to analyses and tools:

Analysis

2010-01-04: Sophisticated, targeted malicious PDF documents exploiting CVE-2009-4324  (embedded binaries)
2010-01-07: Static analysis of malicous PDFs (Part #2) (getAnnots, arguments.callee)
2010-01-09: PDF Obfuscation (variable substitution, LuckySploit, CVE 2008-2992)
2010-01-13: Generic PDF exploit hider. embedPDF.py and goodbye AV detection
2010-01-14: PDF Obfuscation using getAnnots() (getAnnots, arguments.callee, Neosploit)
2010-02-15: Filling Adobe's heap (Javascript, ActionScript and PDF Images)
2010-02-18: Malicious PDF trick: getPageNthWord
2010-02-21: Analyzing PDF exploits with Pyew
2010-03-01: Analyzing PDF Files (getPageNthWord, getPageNumWords)
2010-04-08: JavaScript obfuscation in PDF: Sky is the limit (getAnnots,arguments.callee)

More about the JailbreakMe PDF exploit

Today has been released the source code of the Jailbreakme exploit, so maybe this explanation comes a bit late. In the update of the previous post about this subject I knew that I was right about the overflow in the arguments stack when parsing the charstrings in the Type 2 format, so here is a little more info.

After decoding the stream of the object 13 we can see the following bytes (talking about this file):

cff_bytes

The selected bytes are the important ones for this exploit because the overflow occurs when parsing them. Like I mentioned, the Type 2 format is composed of operands, operators and numbers, and use the stack to push and pop values. This stack has a maximum size of 48 elements. We can understand better the meaning of these bytes with this tips:

 

About the JailbreakMe PDF exploit

Some days ago Comex published his JailbreakMe for the new iPhone 4 in the Defcon 18. The interesting thing is that in order to root the device he used a PDF exploit for Mobile Safari to execute arbitrary code and after this another kernel vuln to gain elevated privileges. I've being taking a look at the PDF files with peepdf and these are my thoughts about it.

The PDF file itself has no many objects and only one encoded stream:

The stream is encoded with a simple FlateDecode filter, without parameters, and if we decode its content we can see this strings, related to the JailbreakMe stuff:
 
As this object seems to contain the vulnerability we are looking for we'll take a closer look to this stream and what this is for:
 

Hiding information in a PDF

I'm gonna stop writing about actions in PDFs to begin with the filters that can be applied to the stream objects. An stream object is composed by a dictionary followed by the real content between the words stream and endstream. Within this dictionary are defined the stream properties like size, filters to apply in order to decode/decompress it or the file name in the case of the stream is located in an external file.
 

As you suppose, a way to hide information in a PDF file is applying to it one or more filters in order to avoid identifying it easily and putting it hard to extract the real content. In fact this is an usual technique in most of the malicious files that try to exploit some of the latest vulnerabilities.

My HelloWorld PDF

Before I continue with the different actions we can perform within a PDF file I'm gonna create a simple PDF file which we can modify easily. If you open a PDF with any text editor you'll see a lot of objects and elements that can confuse you a bit. In order to avoid this let's make a PDF document from scratch with a text editor, without all the unnecessary elements.

We must begin knowing which of the PDF elements are obligatory and must be present in our file. I've written some weeks ago about the physic and logic structure of these types of documents so I'll only enumerate what we'll need:

Actions in the Portable Document Format (PDF)

The PDF format is becoming more and more (in)famous due to the lately published vulnerabilities in Adobe products allowing the execution of arbitrary code in the system. Now I don't want to write about these malicious files but I'll do it in future posts.

After the brief comments about the objects we can find in a document of this type and its physic and logic structure I'm going to follow with the actions that can be executed in background. The PDF files aren't static documents but it's possible to specify some kind of programming depending on the user actions. This is where the security problem arises and that becomes a simple PDF in a potential malcode with high probabilities of being executed.

A PDF action is a dictionary object which can contain the following elements:

  • /Type: it's optional and it's used to specify the object type of the dictionary. In this case it's Action.
     
  • /S: it's an obligatory element that defines the type of the action we want to do.
     
  • /Next: it's optional too and specifies the next action or actions to be executed.

 

 

Portable Document Format (PDF) Basics

Some months ago in the Black Hat Europe, Eric Filiol gave a talk about the functionalities of the PDF format. Filiol said that thanks to some features a simple PDF could become malcode executing the attacker instructions. Besides this, the exploitation of vulnerabilities in this type of documents is more and more usual nowadays. This is why I'm going to write about the basics of the PDF structure and how it works internally. Maybe this can be boring but I promise you that next posts about this subject will be more practical;) To make it more enjoyable you can open a PDF file in a text or hexadecimal editor and take a look at what I mention in the next paragraphs.

A PDF file consist of multiple objects connected between them. This objects can belong to one type from eight possible values: boolean, integer and real numbers, text strings, names, arrays, dictionaries, streams and nulls. Apart of the "known" types, names are a kind of tag for the different elements that compose an object, dictionaries, delimited by "<<" and ">>", are a collection of pairs key-value, and streams, delimited by "stream" and "endstream", are bytes sequences, an information flow that the PDF readers can read incrementally, unlike the normal text strings. All the objects can be declared as indirect objects, assigning them an id to be referenced in any part of the file. This type of objects are delimited by the words "obj" and "endobj".

The physic structure of a PDF file is divided in header, body, cross references table and trailer:

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