According to Russian security firm Kaspersky Lab and Budapest-based CrySyS Lab, which both discovered the attack code -- named MiniDuke -- the campaign appears to remain active, because recovered malware used by attackers was created as recently as Feb. 20.
"To compromise the victims, the attackers used extremely effective social engineering techniques which involved sending malicious PDF documents to their targets," according to an overview of MiniDuke published by Kaspersky Lab. "The PDFs were highly relevant and well-crafted content that fabricated human rights seminar information and Ukraine's foreign policy and NATO membership plans."
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The malicious PDFs exploited a bug, since patched, in Adobe Reader versions 9, 10 and 11, which allowed the attackers to bypass Reader's sandbox and install a small dropper, or downloader, onto the PC that gives an attackers a backdoor for remotely accessing the now-compromised system.
The attack used same the zero-day vulnerability in Adobe Reader discovered by FireEye and first publicly detailed on Feb. 12, after the security firm spotted malicious PDFs disguised as a Turkish visa application. But it's not clear if MiniDuke was launched by the same group, or whether it just purchased a crimeware toolkit from the same vendor that included an exploit for the vulnerability.
Interestingly, each MiniDuke backdoor is custom coded to work only on the targeted machine, meaning if it's moved to a different PC it won't execute. "This downloader is unique per system and contains a customized backdoor written in Assembler," said Kaspersky Lab. "When loaded at system boot, the downloader uses a set of mathematical calculations to determine the computer's unique fingerprint, and in turn uses this data to uniquely encrypt its communications later."
"The backdoor is written in 'old school' assembler and is tiny by current standards -- only 20 KB," according to "The MiniDuke Mystery: PDF 0-day Government Spy Assembler 0x29A Micro Backdoor" research report released Wednesday by Kaspersky and CrySys Lab. "This is most unusual for modern malware, which can be several megabytes in size."
After infecting a PC, the attack code first checks to see if it's infected a desired system. If so, then the PC will surreptitiously contact Twitter accounts created by MiniDuke command-and-control (C&C) servers, which contain tweets which list encrypted URLs -- in the form of hash tags -- to which the infected PC can connect to receive further instructions. These instructions are received in the form of GIF files that are "disguised as pictures that appear on a victim's machine," according to Kaspersky Lab, and enable the downloader to then grab another executable -- one recovered sample was a 300KB file disguised as a GIF -- from a server in either Panama or Turkey. This larger piece of malware then serves as a platform for conducting cyber-espionage, including not just copying and removing files, but also running new malware and spreading malware onto other systems connected to the same network.
The malware includes backup capabilities in the event that Twitter can't be reached or the malnet's Twitter accounts get deleted, such as using Google to search for encrypted URL strings. "This model is flexible and enables the operators to constantly change how their backdoors retrieve further commands or malcode as needed," said the Kaspersky report.
The malware's creators have tried to make their attack code difficult to detect, in part by using disguised JavaScript. "The available malware samples are highly obfuscated, and compiled by a polymorphic compiler. The attackers were able to produce new variants with only a few minutes difference between compile times. Therefore, the number of distinct samples could be very large," said a blog post from the CrySyS lab, which also released a report that includes more detailed information on the malware, as well as tips for how to spot PCs that have been infected.
Based on the logs of command-and-control servers accessed by researchers, MiniDuke has been used only in a small number of targeted attacks. To date, just 59 infected systems have been found in 23 countries, including the United States and much of Europe, as well as Brazil, Israel, Japan, Romania, the Russian Federation and Ukraine.
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