Backdoor.FTPserver

SUMMARY

Backdoor.FTPserver is a general class of Trojan Horse that uses an FTP server as a backdoor into your system.

Protection

• Virus Definitions (LiveUpdate™ Weekly) August 1, 2003
• Virus Definitions (Intelligent Updater) July 31, 2003

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: Low
• Threat Containment: Easy
• Removal: Easy

Damage

• Damage Level: Low
• Payload: Allows remote access to filesystem.
• Deletes Files: Files may be deleted.
• Modifies Files: Files may be modified.
• Releases Confidential Info: Files containing confidential information may be transferred.

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

These Trojan Horses are often based on legitimate software (Serv-U is a common example of a legitimate server used for this purpose), but they have been modified in various ways to elude detection or to give them additional functionality.

Where most backdoor Trojans control a system by accepting commands and directly executing them (for instance, by "Open CD Tray" or "Shut down Windows"), an FTP server is limited in its ability to transfer files to and from your computer.
This could allow an attacker to:

• Steal confidential information
• Delete files
• Install arbitrary software
• Modify system files

Recommendations

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.

REMOVAL

The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

1. Disable System Restore (Windows Me/XP).
2. Update the virus definitions.
3. Run a full system scan and delete all the files detected as Backdoor.FTPserver.
4. Delete the value that was added to the registry.
   

For specific details on each of these steps, read the following instructions.

1. Disabling System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.

Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.

Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.

For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:

• "How to disable or enable Windows Me System Restore"
• "How to turn off or turn on Windows XP System Restore"

For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article, "Antivirus Tools Cannot Clean Infected Files in the _Restore Folder," Article ID: Q263455.

2. Updating the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:

• Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
• Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).
  
  The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

3. Scanning for and deleting the infected files

1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
   • For Norton AntiVirus consumer products: Read the document, "How to configure Norton AntiVirus to scan all files."
   • For Symantec AntiVirus Enterprise products: Read the document, "How to verify that a Symantec Corporate antivirus product is set to scan all files."
2. Run a full system scan.
3. If Backdoor.FTPserver is detected, write down the name and location of the file before you delete it. You will need this information in the sections that follow.

NOTE:
In some cases, because the file that was detected as Backdoor.FTPserver is in use by Windows, you will not be able to delete it. If this happens, restart the computer in Safe mode. All the Windows 32-bit operating systems, except for Windows NT, can be restarted in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."

Windows NT:
To end the Trojan process:
1. Press Ctrl+Alt+Delete once.
2. Click Task Manager.
3. Click the Processes tab.
4. Double-click the Image Name column header to alphabetically sort the processes.
5. Scroll through the list and look for any of the files you have written down.
6. If you find any of the files, click it, and then click End Process.
7. Exit the Task Manager.

4. Deleting the value from the registry

CAUTION: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified keys only. Read the document, "How to make a backup of the Windows registry," for instructions.

1. Click Start, and then click Run. (The Run dialog box appears.)
2. Type regedit, and then click OK. (The Registry Editor opens.)
3. Select the following key:
   
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRun
   
   
4. Refer to the list of infected files that you created while following the instructions in the previous section. In the right pane, look at the entries in the Name and Data columns.
5. If you find an entry that refers to a file that was detected as infected, select the entry, press Delete, and then click Yes to confirm.
6. Select the following key:
   
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRunServices
   
   
7. Refer to the list of infected files that you created while following the instructions in the previous section. In the right pane, look at the entries in the Name and Data columns.
8. If you find an entry that refers to a file that was detected as infected, select the entry, press Delete, and then click Yes to confirm.
9. Exit the Registry Editor.
   
   

5. Editing Windows startup files (Windows 95/98/Me only)

NOTES:
• The instructions in this section applies only to Windows 95/98/Me. It is not necessary to do this if you are running Windows NT/2000/XP.
• For Windows Me users only: Due to the file-protection process in Windows Me, there is a backup copy of the file you are about to edit in the C:WindowsRecent folder. We recommend that you delete this file before you continue with the steps in this section. To do so using Windows Explorer, go to C:WindowsRecent, 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.

1. Click Start, and then click Run.
2. Type the following, and then click OK.
   
   edit c:windowswin.ini
   
   (The MS-DOS Editor opens.)
   
   NOTE: If Windows is installed in a different location, make the appropriate path substitution.
   
   CAUTION: The steps that follow instruct you to remove text from the load= and run= lines of the Win.ini file. If you are using older programs, they may load at startup from one of these lines. The Trojan can add lines such as load=c:windows emppkg2350.exe or run=hpfsched <blank spaces> msrexe.exe. (In this example, hpfsched is a legitimate program, but msrexe.exe is part of the Trojan.)
   
   If you are sure that the text that is contained in these lines is for programs that you normally use, then we suggest that you do not remove it. If you are not sure, but the text does not refer to the file names that you wrote down earlier, then you can prevent the lines from loading by placing a semicolon in the first character position of the line. For example:
   
   ; run=accounts.exe
   
   
3. Locate the load= line within the [windows] section of the Win.ini file; it is usually located near the top of the file.
4. Position the cursor immediately to the right of the equal (=) sign.
5. Press Shift+End to select all of the text to the right of the equal sign, and then press Delete.
6. Repeat steps 3 to 5 for the run= line, which is usually beneath the load= line.
7. Click File, click Exit, and then click Yes when you are prompted to save the changes.
8. Click Start, and then click Run.
9. Type the following, and then click OK.
   
   edit c:windowssystem.ini
   
   (The MS-DOS Editor opens.)
   
   NOTE: If Windows is installed in a different location, make the appropriate substitution.
   
   
10. Locate the shell=explorer.exe line within the [boot] section of the System.ini file; it is usually located near the top of the file.
11. Position the cursor immediately to the right of explorer.exe.
12. Press Shift+End to select all of the text to the right of explorer.exe, and then press Delete.
    
    NOTE: Some computers may have an entry other than explorer.exe after shell=. If this is the case, and you are running an alternate Windows shell, then change this line to shell=explorer.exe for now. You can change it back to your alternate shell after you have finished this procedure.
    
    
13. Click File, click Exit, and then click Yes when you are prompted to save the changes.