Paper: Script in a lossy stream

Posted by   Virus Bulletin on   Mar 2, 2015

Dénes Óvári explains how to store code in lossily compressed JPEG data.

Malformed PDFs have become a common way to deliver malware. Naturally, when this started to happen, anti-virus products began scanning inside PDF files for traces of malicious code and, equally naturally, malware authors started to obfuscate that code to circumvent scanners.

Not everything can be used to store code though. Data streams compressed using lossy compressors like JPXDecode and DCTDecode are deemed unsuitable for storing any kind of code. After all, the lossy compression means one should not be able to retrieve an exact copy of the uncompressed data. For performance reasons, scanners therefore usually ignore this data.

But are they right to do so? Today, we publish a paper by CSIS researcher Dénes Óvári, who found a way to store code as a JPEG image using the DCTDecode filter. His trick, which he explains in the paper, was to encode the data as a greyscale JPEG image, so that no rounding occurs when the images is converted from the RGB to the YCbCr colour space.

To show that this is not merely a theoretical possibility, Dénes wrote a proof-of-concept where a simple (and in this case harmless) piece of JavaScript code was thus embedded inside a PDF.

You can read the paper here in HTML format or here as a PDF. Remember that all content published by Virus Bulletin can be read free of charge, with no registration required.

Both Adobe and anti-virus vendors were given advance notification of the publication of this paper.

Posted on 2 March 2015 by Martijn Grooten

twitter.png
fb.png
linkedin.png
hackernews.png
reddit.png

 

Latest posts:

VBSpam tests to be executed under the AMTSO framework

VB is excited to announce that, starting from the Q3 test, all VBSpam tests of email security products will be executed under the AMTSO framework.

In memoriam: Prof. Ross Anderson

We were very sorry to learn of the passing of Professor Ross Anderson a few days ago.

In memoriam: Dr Alan Solomon

We were very sorry to learn of the passing of industry pioneer Dr Alan Solomon earlier this week.

New paper: Nexus Android banking botnet – compromising C&C panels and dissecting mobile AppInjects

In a new paper, researchers Aditya K Sood and Rohit Bansal provide details of a security vulnerability in the Nexus Android botnet C&C panel that was exploited in order to gather threat intelligence, and present a model of mobile AppInjects.

New paper: Collector-stealer: a Russian origin credential and information extractor

In a new paper, F5 researchers Aditya K Sood and Rohit Chaturvedi present a 360 analysis of Collector-stealer, a Russian-origin credential and information extractor.

We have placed cookies on your device in order to improve the functionality of this site, as outlined in our cookies policy. However, you may delete and block all cookies from this site and your use of the site will be unaffected. By continuing to browse this site, you are agreeing to Virus Bulletin's use of data as outlined in our privacy policy.