'Steganography' Obsfucation Hides Old PDF Exploits From Antivirus Tools

EdgeSpot has found two new obsfucation methods to hide old PDF exploits from various antivirus tools.

Security firm EdgeSpot has been looking for PDF file exploits lately. Now, the firm's researchers have found two interesting ones.

The first is one that uses the PDF JS API this.getPageNumWords() and this.getPageNthWord() to "read" the Javascript contents on the PDF page. It executes the Javascript via eval(box).

This exploit is very similar to CVE-2013-3346, which was not obfuscated at that time of first discovery. Because of the obsfucation, it will bypass antivirus detection.

The obsfucation method that EdgeSpot researchers found could be used to hide other exploits besides this one.

After finding the first one, a second method of obsfucation was found a week later -- and it is potentially much more powerful.

(Source: iStock)
(Source: iStock)

It hides inside images in PDF files. This "steganography" method seems to be designed to once again hide from detection by AV engines. Specifically, researchers found that two layers of obfuscation were used in this exploit. The first layer is what they have previously discussed -- the method of "this.getPageNumWords()" and "this.getPageNthWord()". The exploit uses "this.getPageNumWords()" and "this.getPageNthWord()" to read and execute the Javascript hidden as “content."

The second layer is new and contains "Javascript content."

The API references say that these two APIs, working together, are used to read the stream of an image named "icon" which stored in the PDF file.

Looking at their sample’s Javascript code, researchers figured out that the code's function is to read and decode the "message" hidden in the icon's stream. Once it read the "message" successfully, it executes the "message" as Javascript code, via “eval(msg)”.

The icon stream, which is simply named "icon," could be saved as a "jpg" file and viewed in image viewer without problem.

This led the researchers to conclude that the attacker likely copied a project/technique called "steganography.js", which is open sourced. The project was developed working on browsers.

EdgeSpot researchers also believe the person or persons behind the PDF samples made their innovation as they successfully leveraged the technique in PDF format. They could not find any information mentioning this technique in PDF exploits previously, so they think this is the first time that the "steganography" technique has been used to hide PDF exploits.

By using this technique, all streams look normal, all images are viewable, and everything looks legitimate. This can probably explain why almost all AV engines missed it.

Old exploits can be dressed up to hide themselves from detection. Security teams have to be aware that such mutation may occur, and adjust their own techniques accordingly.

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— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.