Worm:W32/Morto.A propagates through Remote Desktop Services on Windows servers by brute-forcing the login credentials of the server.
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Worm:W32/Morto.A can be viewed as having three components: the dropper, the loader, and the payload.
Upon execution, it sets the data of the registry value HKLM\SYSTEM\WPA\ie to its own full path, then deletes the registry key, HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\RunMRU.
It then saves a copy of the embedded encrypted data to the registry value, HKLM\System\Wpa\md. This encrypted data is actually the payload of the malware.
It then drops a DLL component in the Windows directory named clb.dll (which is the loader) and executes regedit in the Windows run command, then terminates.
The clb.dll or the loader gets triggered when the regedit command was executed. This happens because regedit normally loads a normal Windows DLL component from the %systemdir% folder named clb.dll. Due to the nature of the Windows DLL loading mechanism, the clb.dll file that was dropped by the malware in the Windows directory gets loaded instead of the one in the %systemdir% directory.
As it is the malicious DLL that gets loaded, the regedit command does not show any graphic user interface (GUI) as it normally does. It decrypts and loads the encrypted payload saved at HKLM\System\Wpa\md registry value. This is when the payload takes control.
It renames clb.dll to clb.dllbak so regedit will execute as normal and will not appear to be suspicious to the user.
When the loader is triggered by rundll32.exe, it tries to read the following file-like objects: \\tsclient\a\moto and \\tsclient\a\RIR1.
The malware also does the following modifications:
The payload contains the worm propagation routine and infect other system using exposed Remote Desktop Protocol (RDP) Services. It will brute force the administrator login using hardcoded password list and proceed with infection after the successful login is achieved.
It does this to the computers connected to the local subnet and publicly exposed RDP services in the internet.
On successful login, it will attempt to copy and execute the loader component and a registry file to the server, with the names \\tsclient\a\a.dll and \\tsclient\a\r.reg, respectively.
The registry file contains these entries:
Windows Registry Editor Version 5.00
Note: <Drive_letter> consists of "C" to "I" drive. eg: C:, D:.
Since the loader is expecting rundll32.exe to trigger itself, this seems to give the rundll32.exe Administrator rights, as well as the loader.
The malware also monitors processes related to antivirus and security software and terminates them if the following strings are found:
The malware also attempts to communicate to: