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Malware Decompiler Tool Goes Open Source

Avast's RetDec machine-code decompiler now available for free on Github.

Anti-malware vendor Avast has donated its homegrown malware decompiler tool to the open-source community.

Avast's RetDec basically converts a piece of malware into a higher-level programming language and helps malware analysts unmask the inner workings and functions of its code. "It turns it into something that looks like the original source code," says Jakub Kroustek, threat lab team lead at Avast. "It's much easier" and more efficient to sleuth just what the malware can do when it's decompiled, he notes.

"We're facing millions of new samples of malware each day. We're not deeply analyzing all of them, but by using decompilation we can handle some" more closely and quickly, says Kroustek, founder of the tool.

Researchers at Avast used the tool to decompile ransomware strains including Apocalypse, BadBlock, Bart, CrySIS, and TeslaCrypt, and then offer free decryption tools for the ransomware.

Decompiler tools provide static analysis of code, where researchers don't actually run the code but instead study it, up close. Dynamic analysis is another method, where a researcher executes malicious code in the safety of a sandbox environment to study how it runs in action. "Sometimes it's right to use a sandbox, and other times it's beneficial to use a decompiler," he says. "In my case, I usually use both. When you're fighting bad guys, you trying using every leverage" you can, he says.

RetDec was first created in 2011 by researchers at the Czech Republic's Brno University of Technology and AVG Technologies, and the tool became Avast's last year after it acquired AVG. Kroustek says Avast hopes to get other security experts to help further its development as an open source tool, which is aimed at researchers and reverse-engineers.  

Decompilation tools are nothing new. There are commercial products, which can be pricey and limited in customization, Kroustek says, while there are other open-source decompiler tools such as DCC, Boomerang, and Snowman, for example.

"While good decompilation tools are available that deliver good results, many are paid products, however, these cannot be easily extended with custom features," he says. "On the other hand, users can utilize existing, free, open-source decompilers, but these do not always achieve proper stability, code readability and quality."

John Bambanek, threat systems manager with Fidelis Cybersecurity, who also teaches at the University of Illinois at Urbana-Champaign, says an open-source decompiler such as Avast's can be especially helpful for academia. "I have limited funds and buying a bunch of IDA [Hex-Rays decompiler product] seats isn't going to happen. With something open-source, assuming it can get the job done, [the tool can] "provide a great resource for me to produce more reverse engineers," he says.

Avast says the now open-source tool works on multiple architectures, file formats, and operating systems, and can be used for more than decompilation, too. It uses C and Python-type language for output, and runs on Linux and Windows platforms. RetDec source code and related tools are available now on GitHub, under an MIT license.

"If someone isn't focused on decompilers, he or she can just use the libraries for detection of particular patterns" in the malware, for example, Kroustek says.

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