BackDoor.G | 
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The BackDoor.G Trojan is distributed as a single executable, the installer.
Distribution
When the installer is run, it does the following:
- Drops an executable loader program into the \Windows folder.
- Drops a server .dll file into the \Windows\System folder.
- Modifies either Win.ini file or the registry so that the loader is executed when you start Windows.
When the loader is run, it loads the server into memory. Once the server is in memory, it can allow unauthorized access to your computer. A client program can then be run from a remote location to make use of this access.
Symantec Security Response has determined that there is very little risk associated with this Trojan. To gain access to your computer, the intruder must know that the server has been loaded and is running properly.
There are currently two known variants:
Variant 1
- Installer: Data2.exe
- Loader: Tinurak.exe
- Server: Watching.dll
This variant adds a the value named Kernel16 to the following registry key:
HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/RunServices
Variant 2
- Installer: Window.exe
- Loader: Nodll.exe
- Server: Lmdrki_33.dll
This variant adds the line run=nodll to the Win.ini file.
Both variants of BackDoor.G listen on the following three ports:
Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":
- Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
- If a blended threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
- Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services (for example, all Windows-based computers should have the current Service Pack installed.). Additionally, please apply any security updates that are mentioned in this writeup, in trusted Security Bulletins, or on vendor Web sites.
- Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
- Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
- Isolate infected computers quickly to prevent further compromising your organization. Perform a forensic analysis and restore the computers using trusted media.
- Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
Delete files that are detected as BackDoor.G, and remove either the value that the Trojan added to the registry or the line that it added to the Win.ini file.
To remove the Trojan files:
- Run LiveUpdate to make sure that you have current virus definitions.
- Start Norton AntiVirus (NAV), and make sure that NAV is configured to scan all files. For instructions on how to do this, read the document How to configure Norton AntiVirus to scan all files.
- Run a full system scan.
- Delete all files that are detected as BackDoor.G.
To edit the registry:
CAUTION: We strongly recommend that you back up the registry before you make any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify only the keys that are specified. Read the document How to back up the Windows registry for instructions.
- Click Start, and click Run. The Run dialog box appears.
- Type regedit and then click OK. The Registry Editor opens.
- Navigate to the following key:
HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/RunServices
- In the right pane, delete the following value:
Kernel16
- Click Registry, and then click Exit.
To edit the Win.ini file:
NOTE: (For Windows Me users only) Due to the file-protection process in Windows Me, a backup copy of the file that you are about to edit exists in the C:\Windows\Recent folder. We recommend that you delete this file before you continue with the steps in this section. To do this using Windows Explorer, go to C:\Windows\Recent, and in the right pane select the Win.ini file and delete it. It will be regenerated as a copy of the file that you are about to edit when you save your changes to that file.
- Click Start, and click Run.
- Type the following, and then click OK.
edit c:\windows\win.ini
The MS-DOS Editor opens.
NOTE: If Windows is installed in a different location, make the appropriate path substitution.
- In the [windows] section of the file, look for a line that is similar to the following:
run=nodll
- If the line exists, select the entire line. Be sure that you do not select any other text, and then press Delete.
- Click File, and click Save.
- Click File, and click Exit.
Write-up by: Wason Han and Darren Kessner
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