Over the past several months, Cisco Talos has been monitoring various malware distribution campaigns leveraging the malware loader Brushaloader to deliver malware payloads to systems. Brushaloader is currently characterized by the use of various scripting elements, such as PowerShell, to minimize the number of artifacts left on infected systems. Brushaloader also leverages a combination of VBScript and Powershell to create a Remote Access Trojan (RAT) that allows persistent command execution on infected systems.
Brushaloader is an evolving threat that is being actively developed and refined over time as attackers identify areas of improvement and add additional functionality. We have identified multiple iterations of this threat since mid-2018. Most of the malware distribution activity that we observe associated with Brushaloader leverages malicious email campaigns targeting specific geographic regions to distribute various malware payloads, primarily Danabot. Danabot has already been described in detail here and here, so this post will focus on the analysis of Brushaloader itself. Talos has recently identified a marked increase in the quantity of malware distribution activity associated with Brushaloader, as well as the implementation of various techniques and evasive functionality that has resulted in significantly lower detection rates, as well as sandbox evasion.
The advanced command-line auditing and reporting available within ThreatGrid make analyzing threats such as Brushaloader much more efficient. Threats such as Brushaloader demonstrate the importance of ensuring that PowerShell logging is enabled and configured on endpoints in most corporate environments.