Well-Known and advanced threat actors groups TA505 APT distributing a new variant of ServHelper malware that distributed via weaponized Excel 4.0 macro to open a backdoor & steal sensitive information from infection system.
TA505 threat actors associated with various high profile cyber attackers including infamous Dridex banking trojan and Locky ransomware, delivered through malicious email campaigns.
TA505 group mainly targeting various institutions and organizations including banks, retail businesses, and restaurants.
In this case, Excel 4.0 macro Dropper employed by an attacker to deliver the ServHelper Backdoor and the payload legitimately singed with a valid digital signature to evade the detection.
ServHelper Malware Infection Process
Initially malicious Excel sheet being delivered via malspam emails, once the victims open the document, Excel 4.0 macro is executed and its calls the msiexec.exe in order to download and execute the ServHelper payload.
ServHelper installer singed with a valid digital signature and the The certificate used to sign the malware was issued to “MASLAK LTD” of Uxbridge.
According to the Deep Instinct research, “Once the dropped payload is executed, it will drop a DLL file contained in the installer to \%TEMP%\xmlparse.dll, and use rundll32.exe to call the DLL’s exported function “sega”.
The malware will then write a base64 encoded PowerShell script (which is contained in xmlparse.dll as a resource) to \%TEMP%\enu1.ps1 and execute it.”
Later the malware write the base64 encoded PowerShell script for
reconnaissance purposes, as well as it checks if a machine is part of a domain and if the user has Admin privileges or is part of the Admin Group.
Finally, ServHelper communicates with its C2 to receive the following command from the attacker,
- shell – execute a shell (cmd.exe)command and return its output
- loaddll –download a DLL file and load it using rundll32.exe
- persist – write an auto-run registry entry at HK_CU\Software\Microsoft\Windows\CurrentVersion\Run\ as “Intel Protect”, returns “persistence established” if successful.
- slp – enter sleep mode
- selfkill – remove the malware from the infected machine
“TA505 is a highly advanced global threat actor. It employs a vast array of sophisticated, constantly developed malware for different purposes, for which it exploits the most recently discovered and publicized weak points.” Deep instinct said.
Indicator of Compromise
Excel 4.0 macro Dropper
ServHelper core DLL
Encoded reconnaissance PowerShell script