The threat actors use the Log4j RMI (Remote Method Invocation) exploit variant to force vulnerable devices to load and execute a Java class from an attacker-controlled remote server.
When executed, the Java class will first attempt to download and launch an HTA file from various URLs, which will install the Dridex trojan.
If it cannot execute the Windows commands, it will assume the device is running Linux/Unix and download and execute a Python script to install Meterpreter.
Running Meterpreter on a Linux box will provide the threat actors with a remote shell that they can use to deploy further payloads or execute commands.
The Dridex threat actors are known for using racial and religious slurs in their file names and URLs.
On Windows, the Java class will download an HTA file and open it, which will cause a VBS file to be created in the C:ProgramData folder.
This VBS file acts as the main downloader for Dridex and has been seen previously in other Dridex email campaigns.
When executed, the VBS file will check if the user is part of a Windows domain by checking various environment variables.
If the user is part of a domain, the VBS file will download the Dridex DLL and execute it using Rundll32.exe, as shown below.
As previously said, if the original Java class exploit is unable to launch the Windows commands, it will assume the operating is a Unix/Linux device and download an ‘m.py’ python script instead.
The above script contains a base64 encoded script that will be executed to install Meterpreter, a pentesting tool that provides a reverse shell back to the threat actors.
Using Meterpreter, the threat actors can connect to the compromised Linux server and remotely execute commands to spread further on the network, steal data, or deploy ransomware.
With Log4j exploited by threat actors to install a wide range of malware, it comes as no surprise that the more active malware operations would begin to target the vulnerability.
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