Experts found a new TrickBot module (rdpScanDll) built for RDP bruteforcing operations

A new variant of the TrickBot malware is targeting telecommunications organizations in the United States and Hong Kong.

Security experts from Bitdefender recently discovered a new TrickBot variant that is targeting telecommunications organizations in the United States and Hong Kong.

TrickBot is a popular banking Trojan that has been around since October 2016, its authors have continuously upgraded it by implementing new features. For example, in February 2019 Trend Micro detected a variant that includes a new module used for Remote App Credential-Grabbing.

This new variant includes a module dubbed rdpScanDll to launch remote desktop protocol (RDP) brute-force attacks against a list of victims.

“The new module was discovered on January 30, and its main functionality is to perform bruteforce operations on a list of targets. The modus operandi is similar to that of other plugins.” reads the report published by Bitdefender. “The TrickBot executable will download the plugin and its configuration file (from one of the available online C&Cs) containing a list of servers with whom the plugin will communicate to retrieve commands to be executed. TrickBot will load the plugin, executing the “start” and “control” exported functions, passing the configuration file as an argument for the last mention function.”

The module appears to be under development, but experts pointed out that threat actors already used it to target organizations, mostly in telecoms, education, and financial services sectors.

The module implements three attack modes, named check, trybrute and brute.

The check mode should check for RDP connection on the list of targets, trybrute mode performs a brute-force attack on the list of targeted IPs returned by the /rdp/domains command, and the brute mode appears to be broken. According to the researchers, the problems with the brute mode suggests it is still under development.

Trickbot has a modular structure, it could implement new capabilities by loading specific plugins. During the last 6 months, Bitdefender’s systems were able to retrieve updates for several different active plugins.

Upon the TrickBot infection, the malware awaits commands from the command and control (C&C) server. The Trojan could load specific plugins to execute commands received from the C2.

The downloaded plugins allow the malware to perform lateral movements, reconnaissance, data harvesting, set foothold, exfiltrate data, and perform brute-force attacks.

Researchers retrieved 3,460 IP addresses associated with TrickBot, 2,926 were related to C&C servers, 556 were used to provide new plugins, and 22 used for both functionalities. Experts noticed that around 100 new IPs were added to the infrastructure each month, each IP was used on average 16 days.

The analysis of the distribution of the infections revealed that most of the victims over the past month were in the United States (nearly 30,000), with Spain (10,000) and Canada (3,500) rounding up the top three.

“The new rdpScanDll module may be the latest in a long line of modules that have been used by the TrickBot Trojan, but it’s one that stands out because of its use of a highly specific list of IP addresses.” concludes Bitdefender. “While the module seems to be under development, as one attack mode seems broken, newer versions of rdpScanDll will likely fix this and potentially add new ones,”.

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Pierluigi Paganini

(SecurityAffairs – malware, Trickbot)

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