The shell-code in the Word document decrypts and drops the backdoor's file as CSRSE.EXE to the system's temporary folder and activates it.
After being run, the dropped file in its turn extracts and drops another file to the system. This file is dropped as WINGUIS.DLL to Windows System folder and its DoHook function is activated by the dropper. The dropper then deletes itself from the system.
The dropped DLL file is the main backdoor component. It traps several functions and modifies information that is passed to the user. As a result, the backdoor's file, startup key in the Registry, and process are not visible to the user.
The backdoor creates a startup key for its file in the Registry:
- [HKLM\Software\Microsoft\Windows NT\CurrentVersion\Windows]"AppInit_DLLs" = "%WinSysDir%\winguis.dll"
where %WinSysDir% represents the Windows System folder that by default has the name C:\Windows\System32\.
When active, the backdoor connects to a specified address in order to receive commands from a hacker. The backdoor allows the hacker to do any of the following on an infected computer:
- Create, read, write, delete and search for files and directories
- Access and modify the Registry
- Manipulate services
- Start and kill processes
- Take screenshots
- Enumerate open windows
- Create its own application window
- Get information about infected computer
- Lock, restart or shutdown Windows
- Create a pipe and read files from it
- Start a remote command shell
- Enumerate network resources
The backdoor also creates three empty SYS files in the \drivers\ subfolder of Windows System folder.