A recently discovered PDF exploit used steganography to hide malicious JavaScript code in images embedded in PDF files, according to exploit analysis firm EdgeSpot.

Steganography is a method of hiding information within files, and has been long used by cybercriminals in malicious attacks. The powerful exploit obfuscation technique allows threat actors to generate PDF documents that can bypass the detection of almost all anti-virus engines, EdgeSpot's researchers say. 

The analyzed sample was initially submitted to VirusTotal in October 2017, but only one anti-virus engine was detecting it as an exploit last week, EdgeSpot says. The detection rate didn’t improve by much by the time the researchers completed their investigation.

The sample included two layers of obfuscation, one abusing two methods to read and execute the JavaScript hidden as "content,” and the other leveraging steganography to hide the code in stream-119. 

The attackers used the this.getIcon() and util.iconStreamFromIcon() PDF JS APIs that, when working together, can read the stream of an image named as "icon" stored in the PDF file. 

A "message" hidden in the icon’s stream was read and decoded, and then executed as Javascript code, via "eval(msg)". The malicious code is heavily obfuscated, so as to avoid rising suspicion. 

To apply this technique to their documents, the attackers “likely copied a project/technique called "steganography.js", which is open sourced,” EdgeSpot says.

The project was meant to target browsers, but the attackers likely modified it and successfully used it in the creation of malicious PDF samples. 

“We could not find any information mentioning such technique in PDF exploits before, so we believe this is the first time that the 'steganography' technique is used to hide PDF exploits,” the security researchers say. 

The use of this malicious code obfuscation allows attackers to create PDF documents in which streams look normal and images are viewable, thus making everything seem legitimate. 

“The 'steganography' technique could not only be used to obfuscate this exploit (CVE-2013-3346) but also can be applied to many other PDF exploits including zero-days. We ask security defenders to pay close attention to it,” EdgeSpot concludes. 

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