Fortinet Finds Loader Uses Updated Version of Backdoor

When malware has been around for a while, its authors are faced with the need for its maintenance if they want it to continue to succeed in their criminal activity.

Security firm Fortinet has found traces of how the financially motivated FIN7 group does this kind of maintenance on delivering the Carbanak backdoor malware.

FireEye has already looked at one way that FIN7 does this with its BOOSTWRITE loader tool. Fortinet found that the predecessor BIOLOAD tool is a loader as well, but with a different and singular payload. BIOLOAD is not as easily detected as BOOSTWRITE but has the same end effect.

The malware subverts the normal way that Windows will load a Dynamic Linked Library (DLL), by a technique known as DLL search order hijacking (or binary planting). In this case, the attackers use FaceFodUninstaller.exe. This exists on a clean OS installation starting from Windows 10 RS4 (1803) at the "%WINDR%\System32\WinBioPlugIns" folder. The executable is also dependent on winbio.dll, which is usually found in the parent directory ("%WINDR%\System32").

The uninstaller program is started by a built-in scheduled task named FODCleanupTask, which minimizes the attacker's footprint on the machine and therefore reduces the chances of detection even further.\r\nBIOLOAD has the encrypted payload DLL embedded in it. Unlike BOOSTWRITE, it does not support multiple payloads. Decrypting the payload is done by a simple XOR decryption. This is a key point, because it implies that BIOLOAD is tailor-made for every machine it infects. It will use the machine name to derive the decryption key. This means that it does not need to access a remote server in order to fetch the key.

The attacker places the resultant malicious WinBio.dll in the "\System32\WinBioPlugIns" folder, which is home of the legitimate DLL "winbio".

Fortinet says that, "The samples [that they found] target a 64-bit OS and were compiled in March and July of 2019. BOOSTWRITE targets 32-bit machines and was compiled (and signed) in May 2019."

Fortinet also found that BIOLOAD contained newer builds of the Carbanak backdoor than BOOSTWRITE. They were dated January and April of 2019, according to their timestamps. This version checks to see if another Anti-Virus (AV) is running on the machine, besides Kaspersky, AVG and TrendMicro. The result, however, has no effect on the operations of the backdoor.

BIOLOAD has the characteristic of being the first public case of FaceFodUninstaller.exe being abused as host process by a threat actor.

Also, BIOLOAD shows that the attackers are specifically building infections for each targeted machine as well as somehow obtaining the needed administrative permissions to deploy it. This implies that the group needs to gather other critical information about its targets' networks in order to carry out the attack.

— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.