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Bifrose.UZ

Classification

Category: Trojan

Type: Backdoor

Aliases: Bifrose.UZ, Backdoor.Win32.Bifrose.uz, W32/Bifrose.UZ, Backdoor:Win32/Bifrose!E029, Backdoor.Bifrose.E

Summary


Bifrose.UZ is a backdoor that is dropped from a PPT (Microsoft Power Point) file. The malware was sent to a limited number of computer users via email. The backdoor provides unauthorized access to an infected computer for remote hackers.

Removal


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Technical Details


The backdoor's file is a PE executable that is not compressed in any way. Instead, the file is encrypted with a simple algorithm. Additionally, there is one encrypted DLL file inside the backdoor.

The backdoor's file gets dropped by the PPT file directly to the Windows System folder, named as regvrt.exe, and this file is then started. After being started, the backdoor checks the command line and if it's empty, it starts the file again, but this time with a specific command line option. After that the backdoor decrypts its main code, it performs different self-checks and decrypts data that will be used during the backdoor's operation (for example: remote site name, startup Registry key name, extra backdoor copy name, etc.).

The backdoor copies itself one more time to the Windows System folder, named as rtfmsv.exe, and a startup key value is created for that file in the Windows Registry:

  • [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run]"StartKey" = "%WinSysDir%\rtfmsv.exe"

where %WinSysDir% represents the Windows System folder, which is by default C:\Windows\System32 .

Additionally, the backdoor creates its own keys in the Registry where it stores its data:

  • [HKEY_CURRENT_USER\Software\SKavx]
  • [HKEY_LOCAL_MACHINE\Software\SKavx]

The backdoor deletes the following key from the Registry:

  • [HKEY_CURRENT_USER\SOFTWARE\Microsoft\Active Setup\Installed Components\{A5CDF7EC-751B-46aa-AD69-4005FE080DE9}]

Finally the backdoor decrypts a DLL file that is stored in its body and injects it into the Explorer.exe process. Then the backdoor's own process terminates. The DLL file is the backdoor's main component. It allows a remote hacker to acquire limited access to an infected computer. It also allows the hacker to spy against a user.

The backdoor attempts to connect to the following website in order to receive commands from a remote hacker:

  • pukumalon.8800.org