On November 3, Check Point Research (CPR) released research on Mekotio, a modular banking Remote Access Trojan (RAT) that targets victims in Brazil, Chile, Mexico, Spain, and Peru, and it's now back with new techniques for evading detection. 

In October, 16 people were arrested across Spain in connection with Mekotio and the Grandoreiro Trojans. The individuals are suspected of sending hundreds of phishing emails to spread the Trojan, which was then used to steal banking and financial information. As per local media sources, 276,470 euros were stolen, but 3,500,000 euros worth of transfer attempts were made, which were luckily blocked. 

According to CPR researchers Arie Olshtein and Abedalla Hadra, the arrests simply delayed the transmission of the malware across Spain, and the malware is still spreading since the group probably partnered with other criminal organisations. Mekotio's developers, suspected of being based in Brazil, quickly rehashed their malware with new characteristics aimed to prevent detection after the arrests were revealed by the Spanish Civil Guard. 

The infection vector of Mekotio has remained the same, including phishing emails containing either links to or malicious code. The payload is contained in a ZIP archive attached. However, an examination of more than 100 recent attacks indicated the use of a simple obfuscation approach and a substitution cypher to avoid detection by antivirus software. 

In addition, the developers have included a redesigned batch file with numerous levels of obfuscation, a new PowerShell script that runs in memory to conduct malicious actions, and the use of Themida to safeguard the final Trojan payload — a legitimate application that prevents cracking or reverse engineering. 

Mekotio attempts to exfiltrate login credentials for banks and financial services once it has been installed on a vulnerable machine and will send them to a command-and-control (C2) server controlled by its operators. 

The researchers stated, "One of the characteristics of those bankers, such as Mekotio, is the modular attack which gives the attackers the ability to change only a small part of the whole in order to avoid detection. CPR sees a lot of old malicious code used for a long time, and yet the attacks manage to stay under the radar of AVs and EDR solutions by changing packers or obfuscation techniques such as a substitution cipher."