Backdoor:Win32/Remosh.A is a trojan that sends Windows system configuration details to a remote server and also allows remote access and control of the affected computer.
Installation
Backdoor:Win32/Remosh.A is installed by a dropper trojan such as Backdoor:Win32/Remosh.A.dr and is present as a DLL component located in the Windows system folder. The DLL runs as a service at Windows start. Below is an example of one observed registry modification made by the trojan dropper after installing Backdoor:Win32/Remosh.A to run as a service:
Sets value: "ServiceDll"
With data: "<system folder>\hpmdp093.dll"
In subkey: HKLM\SYSTEM\CurrentControlSet\Services\6to4\Parameters
Note: <system folder> refers to a variable location that is determined by the malware by querying the Operating System. The default installation location for the System folder for Windows 2000 and NT is C:\Winnt\System32; and for XP, Vista, and 7 is C:\Windows\System32.
It contains the following variables in its code; note that the actual values of the variables may change depending on the malware sample:
MUTEXNAME = "NT1630"
SHELLCOMMAND = "shell"
SERVICENAME = "6to4"
SERVICE_DISPLAYNAME = "ASP.NET Services"
SERVICE_DESCRIPTION = ""
DISABLE_IPSEC = "1"
MASTER_HOST = "<string>.is-a-chef.com"
MASTER_PORT = "80"
The malware checks for a mutex named "MUTEXNAME" to verify that it isn't already running. If found, it exits immediately; if not, it creates the mutex.
If the value of "DISABLE_IPSEC" is non-zero, the trojan stops and disables the "PolicyAgent" service, which is the IPSec service.
Win32/Remosh registers itself to run as a service named "SERVICENAME", with a display name "SERVICE_DISPLAYNAME" and a description as "SERVICE_DESCRIPTION". The service is configured so it cannot be stopped, however it does respond to system shutdown requests.
Payload
Allows remote access and control
Every 30 seconds, the malware establishes a TCP connection to "MASTER_HOST" on port "MASTER_PORT" - all subsequent communication is encrypted. The trojan sends system information (computer name, processor information, OS version).
The trojan accepts commands from the server if the server reply starts with "SHELLCOMMAND"; commands include the following:
enumerate terminal services sessions to show who is logged into the machine
enumerate system drives, collecting drive letters, types, free space and volume name
enumerate files by path, allowing the server to browse the contents of the file system
launch an executable remotely
open a remote command shell which allows the server to execute commands
enumerate registry keys
send screenshots from the local system to the server to show what is happening on the system
uninstall itself
move, delete or copy a file
set file attributes on a file
receive a new file from the server.
Additional Information
Note that the file name used by this trojan may vary from sample to sample. This is because this trojan is created using a construction toolkit known as "Gh0st Rat". Therefore, certain aspects of the trojan are author-defined such as the following:
- Mutex name
- Service name
- Service display name
- Service description
- IPSEC options
- Command and control server domain name
- Command and control communication port
Analysis by Aaron Putnam
