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Threat Spotlight: Astaroth – Maze of Obfuscation and Evasion Reveals Dark Stealer

By Nick Biasini, Edmund Brumaghin and Nick Lister.

Executive summary

The threat landscape is littered with various malware families being delivered in a constant wave to enterprises and individuals alike. The majority of these threats have one thing in common: money. Many of these threats generate revenue for financially motivated adversaries by granting access to data stored on end systems that can be monetized in various ways. To maximize profits, some malware authors and/or malware distributors go to extreme lengths to evade detection, specifically to avoid automated analysis environments and malware analysts that may be debugging them. The Astaroth campaigns we are detailing today are a textbook example of these sorts of evasion techniques in practice.

The threat actors behind these campaigns were so concerned with evasion they didn’t include just one or two anti-analysis checks, but dozens of checks, including those rarely seen in most commodity malware. This type of campaign highlights the level of sophistication that some financially motivated actors have achieved in the past few years. This campaign exclusively targeted Brazil, and featured lures designed specifically to tailor to Brazilian citizens, including COVID-19 and Cadastro de Pessoas Físicas status. Beyond that, the dropper used sophisticated techniques and many layers of obfuscation and evasion before even delivering the final malicious payload. There’s another series of checks once the payload is delivered to ensure, with reasonable certainty, that the payload was only executed on systems located in Brazil and not that of a researcher or some other piece of security technology, most notably sandboxes. Beyond that, this malware uses novel techniques for command and control updates via YouTube, and a plethora of other techniques and methods, both new and old.

This blog will provide our deep analysis of the Astaroth malware family and detail a series of campaigns we’ve observed over the past nine to 12 months. This will include a detailed walkthrough of deobfuscating the attack from the initial spam message, to the dropper mechanisms, and finally to all the evasion techniques astaroth has implemented. The goal is to give researchers the tools and knowledge to be able to analyze this in their own environments. This malware is as elusive as it gets and will likely continue to be a headache for both users and defenders for the foreseeable future. This will be especially true if its targeting moves outside of South America and Brazil.


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