ESET researchers discover surprisingly many indicators of close cooperation among Latin American banking trojans’ authors

ESET has published a white paper detailing its findings about interconnectivity of Latin American banking trojan families. The white paper was also published by Virus Bulletin.

For a long time, Latin American banking trojans were looked upon as one group of malware. ESET researchers discovered that is not the case and that, despite having so much in common, multiple distinct malware families can be recognized among these banking trojans. Over the past year, we have been publishing an ongoing blogpost series about Latin American banking trojan malware families. These blogposts focus mainly on the most important and interesting aspects of these families. So far, we have unmasked Amavaldo, Casbaneiro, Mispadu, Guildma, Grandoreiro and Mekotio in this series. In the pieces to come, we will continue with Krachulka, Lokorrito, Numando, Vadokrist and Zumanek.

In this white paper, we look at these families from a higher-level perspective – rather than examining details of each family and highlighting their unique characteristics, we focus on what they have in common. If you’ve been following our series, you may have noticed some similarities between multiple families in our series, such as using the same uncommon algorithm to encrypt strings or suspiciously similar DGAs to obtain C&C server addresses.

The first similarities we spotted are in the actual implementation of these banking trojans. The most obvious one is the practically identical implementation of the banking trojans’ cores – sending notification to operator, periodically scanning active windows based on name or title, and attacking via fake pop-up windows designed carefully in an attempt to lure out sensitive information from victims. Besides that, these malware families share uncommon third-party libraries, string encryption algorithms, and string and binary obfuscation techniques.

However, the similarities do not end there. When analyzing the distribution chains of these malware families, we realized they share the same core logic, too – they usually check for a marker (an object, such as a file or registry key value used to indicate that the machine has already been compromised), and download data in ZIP archives. Besides that, we have observed identical distribution chains ending up distributing multiple Latin American banking trojans. It is also worth mentioning that since 2019, the vast majority of these malware families started to utilize Windows Installer (MSI files) as the first stage of the distribution chain.

Latin American banking trojans share execution methods as well. They tend to bring their own tools bundled in the aforementioned ZIP archives. The two most common methods are DLL side-loading and abusing a legitimate AutoIt interpreter. Additionally, when using the former method, multiple families abuse the same vulnerable applications for that purpose (so-called Bring Your Own Vulnerable Software).

The term “Latin American banking trojan” comes from the region these banking trojans typically target – Latin America. However, since late 2019, we see several of them adding Spain and Portugal to the list of countries they target. Moreover, different families use similar spam email templates in their latest campaigns, almost as if this were a coordinated move as well.

Given so many similarities, one would expect the fake pop-up windows these banking trojans use to be shared too. In fact, the opposite seems to be the case. Even though the windows look similar (since they are designed to fool customers of the same financial institutions), we have not spotted multiple families using identical windows.

Since we do not believe it to be possible that independent malware authors would come up with so many common ideas and we also don’t believe one group to be responsible for maintaining all these malware families, we conclude that these are multiple threat actors closely cooperating with each other. You can find detailed information about the similarities we briefly introduced here, in the whitepaper.

MITRE ATT&CK techniques

In the table below, which is an aggregate of the techniques based on the standard MITRE ATT&CK table, we illustrate many of the features Latin American banking trojans share. It is not an exhaustive list, but rather one that focuses on the similarities. It shows mainly that:

  • phishing is the most common attack vector
  • they heavily rely on scripting languages, mainly VBScript
  • Registry Run key or Startup folder are the most common methods of persistence
  • they all obfuscate either payloads or configuration data in some way
  • they heavily favor DLL side-loading
  • to steal credentials, they tend to use either fake pop-up windows or keyloggers
  • they devote considerable effort to collect screenshots and scan for security software
  • custom encryption algorithms are favored over established ones
  • they do not exfiltrate all harvested data to the C&C server, but use different locations as well

Note: This table was built using version 7 of the MITRE ATT&CK framework.

Initial AccessT1566.001Phishing: Spearphishing Attachment
T1566.002Phishing: Spearphishing Link
ExecutionT1059.005Command and Scripting Interpreter: Visual Basic
T1059.007Command and Scripting Interpreter: JavaScript/JScript
T1059.003Command and Scripting Interpreter: Windows Command Shell
T1059.001Command and Scripting Interpreter: PowerShell
T1047Windows Management Instrumentation
T1059Command and Scripting Interpreter
PersistenceT1547.001Boot or Logon Autostart execution: Registry Run Keys / Startup Folder
T1053.005Scheduled Task/Job: Scheduled Task
Defense EvasionT1140Deobfuscate/Decode Files or Information
T1574.002Hijack Execution Flow: DLL Side-Loading
T1497.001Virtualization/Sandbox Evasion: System Checks
T1218.007Signed Binary Proxy Execution: Msiexec
T1036.005Masquerading: Match Legitimate Name or Location
T1197BITS Jobs
T1112Modify Registry
T1218.011Signed Binary Proxy Execution: Rundll32
T1027.001Obfuscated Files or Information: Binary Padding
T1220XSL Script Processing
Credential AccessT1056.002Input Capture: GUI Input Capture
T1056.001Input Capture: Keylogging
T1555.003Credentials from Password Stores: Credentials from Web Browsers
T1552.001Unsecured Credentials: Credentials In Files
DiscoveryT1010Application Window Discovery
T1518.001Software Discovery: Security Software Discovery
T1082System Information Discovery
T1083File and Directory Discovery
T1057Process Discovery
CollectionT1113Screen Capture
T1115Clipboard Data
Command and ControlT1132.002Data Encoding: Non-Standard Encoding
T1571Non-Standard Port
T1132.001Data Encoding: Standard Encoding
T1568.002Dynamic Resolution: Domain Generation Algorithms
T1568.003Dynamic Resolution: DNS Calculation
ExfiltrationT1048Exfiltration Over Alternative Protocol
T1041Exfiltration Over C2 Channel

As you can see, Latin American banking trojans, while having their differences, have many crucial features in common.

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