Under the hood of Wslink’s multilayered virtual machine

ESET researchers describe the structure of the virtual machine used in samples of Wslink and suggest a possible approach to see through its obfuscation techniques.

ESET researchers recently described Wslink, a unique and previously undocumented malicious loader that runs as a server and that features a virtual-machine-based obfuscator. There are no code, functionality or operational similarities that suggest this is likely to be a tool from a known threat actor.

In our white paper, linked below, we describe the structure of the virtual machine used in samples of Wslink and suggest a possible approach to see through the obfuscation techniques used in the analyzed samples. We demonstrate our approach on chunks of code of the protected sample. We were not motivated to fully deobfuscate the code, because we discovered a non-obfuscated sample.

Under the hood of Wslink’s multilayered virtual machine: Download Research Paper

Obfuscation techniques are a kind of software protection intended to make code hard to understand and hence conceal its objectives; obfuscating virtual machine techniques have become widely misused for illicit purposes such as obfuscation of malware samples, since they hinder both analysis and detection. The ability to analyze malicious code and subsequently improve our detection capabilities is the driving force behind our motivation to overcome these techniques.

Virtualized Wslink samples do not contain any clear artifacts, such as specific section names, that easily link it to a known virtualization obfuscator. During our research, we were able to successfully design and implement a semiautomatic solution capable of significantly facilitating analysis of the underlying program’s code.

This virtual machine introduced a diverse arsenal of obfuscation techniques, which we were able to overcome to reveal a part of the deobfuscated malicious code that we describe in this blogpost. In the last sections of our white paper, we present parts of the code we developed to facilitate our research.

Our white paper also provides an overview of the internal structure of virtual machines in general, and introduces some important terms and frameworks used in our detailed analysis of the Wslink virtual machine.

In an earlier white paper, we described the structure of a custom virtual machine, along with our techniques to devirtualize the machine. That virtual machine contained an interesting anti-disassembly trick, previously utilized by FinFisher – spyware with extensive spying capabilities, such as live surveillance through webcams and microphones, keylogging, and exfiltration of files. We additionally presented an approach for its deobfuscation.

This blogpost consists of excerpts from the Under the hood of Wslink’s multilayered virtual machine white paper; we encourage everyone interested in virtual machines and obfuscation techniques to go through the original white paper, as it contains detailed information on various steps required to see through the obfuscation techniques used in Wslink.

written by Vladislav Hrcka, ESET


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