This blog was authored by Ben Baker, Edmund Brumaghin, and Jonah Samost.

Executive Summary

GozNym is the combination of features from two previously identified families of malware, Gozi and Nymaim. Gozi was a widely distributed banking trojan with a known Domain Generation Algorithm (DGA) and also contained the ability to install a Master Boot Record (MBR) rootkit. Nymaim emerged in 2013 as malware which was used to deliver ransomware and was previously distributed by the Black Hole exploit kit. The code had various anti-analysis techniques, such as the obfuscation of Win32 API calls.

There have been multiple instances in which the source code of the Gozi trojan has been leaked. Due to these leaks it was possible for the GozNym authors to make use of the 'best of breed' methodologies incorporated into Gozi and create a significantly more robust piece of malware which was now capable of utilizing strengthened persistence methods and ultimately becoming a powerful banking trojan.

Given the recent success of the GozNym trojan and the number of targeted attacks seeking to infect victims with this malware, Talos decided to take a deep look at the inner workings of this particular malware family. Talos started by examining the binaries associated with GozNym as well as the distribution mechanisms. Additionally, we were able to successfully reverse engineer the DGA associated with a GozNym command and control (C2) infrastructure and sinkhole that botnet. This gave Talos great visibility into the size and scope of this threat and the number of infected systems beaconing to C2 servers under adversarial control.

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