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Saturday, November 24, 2012

Detecting a MALICIOUS PDF:PDFid @ BACKTRACK 5 R3


1.    Adobe, who gave us the the ever comfortable PDF..thats the "Portable Document Format" in the early 1990's never thought like how this can become a security threat by the simple action of opening it only....yess!!!this post will give a small insight of how things really work behind the scene in execution of a malicious PDF....

2.  So first of all...how a PDF becomes a malicious document?The answer to this question is simple embedding of a JAVA SCRIPT, that is not seen but only executed once a PDF is opened....no antivirus will be able to identify of what malicious thing lies behind a normal PDF that u and me use daily...so if u scan a malicious PDF with your Antivirus,it is veri unlikely to be caught....how do we know then whether a PDF is malicious or not?...thats what this post shows here....I came across a tool known as PDFid in the BACKTRACK R3 that I was running in Virtual Box.

3.   Few lines about the tool....this was developed by Didier Stevens who blogs at http://blog.didierstevens.com/.So this helps us to differentiate between PDF Documents that could be malicious and those that are unlikely to be....The tool is based on the fact that that a  typical PDF File comprises of header, objects, cross-reference table (to locate objects), and trailer.So , if there is a tool that can find out if any one of them is available in this PDF...things can become easier...so like for example...if a PDF that has no purpose of embedding or holding a JS inside it,then a eye brow raise is certain as to why should it be there....so PDFid tool comes to rescue us out of this question...First the typical structure of a PDF with its one line explanation is given below :

“/OpenAction” and “/AA” (Additional Action) specifies the script or action to run automatically.

 “/Names”, “/AcroForm”, “/Action” can also specify and launch scripts or actions.

“/JavaScript” specifies JavaScript to run.

 “/GoTo*” changes the view to a specified destination within the PDF or in another PDF file.

 “/Launch” launches a program or opens a document.

“/URI” accesses a resource by its URL.

“/SubmitForm” and “/GoToR” can send data to URL.

“/RichMedia” can be used to embed Flash in PDF.

“/ObjStm” can hide objects inside an Object Stream.

4. So now I have set up a VB machine running BTR3 that would run this tool and find out if the PDF that I have analyzed is malicious or not? These are the screen shots showing a step by step scene of how u do it....


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5.   So the last screen shows the final result...for those of you who find this little complicated I will upload a video cast of this soon....

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