This is not the first time the malicious XLSB file has been discovered.
As distributed files mostly took the form of XLS or XLSX file format, this calls for increased attention to the future trend.
Upon running the Excel file and clicking the image embedded in it – after enabling macro activates integrated malicious features.
It appears that the design is to avoid detection measures such as an automatic analysis system.
Furthermore, the attacker added ‘protection’ to the Excel file to complicate the file analysis.
Given that the macro sheet can only be viewed after entering the valid password, it would be safe to say that the development process of malware has become even more sophisticated.
hen the user clicks the image, the file creates a malicious file in the ProgramData directory and runs it via WMIC.
The malicious file is a VBScript file under the disguise of an RTF file.
The executed VBScript contains the following code (partial).
The main features of the VBScript are as follows:
Checks %USERDOMAIN% and %LOGONSERVER% info of the user’s system – To check whether it’s domain controller
If it is domain controller, connects to https://cdn.discordapp.com to check whether network connection is available
If it is not domain controller, doesn’t run the rest of the features
Creates malicious DLL file in ProgramData directory
Runs malicious DLL file with Rundll32.exe process C:ProgramDatanianigger.bin DllRegisterServer
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