Backdoor.Exdis

SUMMARY

Backdoor.Exdis is a backdoor Trojan horse that allows a hacker to remotely control a computer through IRC.

Protection

• Virus Definitions (LiveUpdate™ Weekly) November 12, 2003
• Virus Definitions (Intelligent Updater) November 10, 2003

Threat Assessment

Wild

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

Damage

• Damage Level: Medium
• Payload: Opens a backdoor through IRC.
• Releases Confidential Info: Steals passwords; can log keystrokes.
• Causes System Instability: May cause instability due to bugs.

Distribution

• Distribution Level: Low
• Ports: Outgoing connection on TCP port 8080 (variants may use a different port).

TECHNICAL DETAILS

When the Trojan is executed, it performs the following actions:

1. May be copied to the %Windir% folder as a random file name.
   
   

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   Notes:
   • %Windir% is a variable. The Trojan can be found in the Windows installation folder (by default, this is C:Windows or C:Winnt).
   • The name and location of the file may be different, depending on the method by which the Trojan was placed on the computer.

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2. Sets the value:
   
   "(Default)"="ABCDEF %1 %*" (where ABCDEF represents the filename of the Trojan)
   
   in the registry key:
   
   HKEY_CLASSES_ROOTexefileshellopencommand
   
   
3. If the operating system is Windows 2000/XP/NT, it will create a service for the Trojan and set it to automatically run. Known variants of this Trojan have used the following service names, but other names can be selected:
   • xd2s
   • xad2
   • xadx
   • xadz
     
     
4. May create several non-viral files in the %System% folder, for example:
   
   • xad600.bin
   • execmd.xad
   • klg.dat
   • plg.dat
   • prtxad.bin
   • rasxad.bin
     
     

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     Note: %System% is a variable. The Trojan locates the System folder and copies itself to that location. By default, this is C:WindowsSystem (Windows 95/98/Me), C:WinntSystem32 (Windows NT/2000), or C:WindowsSystem32 (Windows XP).

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5. Connects to an IRC server and waits for commands. This backdoor can do the following:
   
   • View and kill processes
   • Upload and download files
   • Execute commands
   • Log keystrokes
   • Steal passwords
   • Act as an HTTP proxy
   • Send email from an infected computer

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. Restart the computer in Safe mode or VGA mode.
4. Reverse the changes made to the registry.
5. Scan for and delete files detected as Backdoor.Exdis.

For 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"

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Note: When you are completely finished with the removal procedure and are satisfied that the threat has been removed, re-enable System Restore by following the instructions in the aforementioned documents.

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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. Restarting the computer in Safe mode or VGA mode
Shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode or VGA mode.

• For Windows 95, 98, Me, 2000, or XP users, restart the computer in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."
• For Windows NT 4 users, restart the computer in VGA mode.

4. Reversing the changes made to the registry

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WARNING: 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.

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1. On the Desktop, double-click My Computer.
   
   
   
2. Make sure that Windows is showing file extentions. In order to check this, do the following:
   
   1. Click the View menu (Windows 95/98/NT) or the Tools menu (Windows Me/2000/XP), and then click Options or "Folder options."
   2. Click the View tab.
   3. Click the Restore Defaults button.
   4. Click OK.
      
      
3. Navigate to the following file, depending on your operating system:
   
   • Windows 95/98/Me/XP:
     C:Windowsegedit.exe
     
     
   • Windows NT/2000:
     C:Winntegedit.exe
     
     
4. Right-click on regedit.exe and select Rename.
   
   
5. Type the following:
   
   regedit.com
   
   
   and then press Enter.
   
   
6. Double click regedit.com.
   
   
7. Navigate to and select the key:
   
   HKEY_CLASSES_ROOTexefileshellopencommand
   
   
8. In the right pane, double-click the (Default) value.
   
   
9. Delete the current value data, and then type:
   
   "%1" %*
   
   That is, type the characters: quote-percent-one-quote-space-percent-asterisk.
   
   
10. The next few steps depend on the operating system you are running.
    
    • If you are running Windows 95/98/Me, skip to step m.
    • Windows 2000/XP/NT only
      Navigate to each of these keys:
      
      HKEY_LOCAL_MACHINESystemControlSet001Services
      HKEY_LOCAL_MACHINESystemControlSet002Services
      HKEY_LOCAL_MACHINESystemCurrentControlSetServices
      
      
11. From each one, delete any of these subkeys that you find:
    
    xd2s
    xad2
    xadx
    xadz
    
    
12. If none of these subkeys are found, the Trojan may be using a different service name. In which case, search the registry for references to the Trojan file:
    
    1. Click Edit > Find.
       
       
    2. In the "Find what" box, type:
       
       ImagePath
       
       
    3. In the "Look at" section, make sure that the Values check box is selected. Deselect the Keys or Data check boxes.
       
       
    4. Click Find Next.
       
       
    5. Allow the search to run until it finds an entry similar to the following:
       
       ImagePath = "%Windir%<the Trojan file name>"
       
       in a registry key similar to the following:
       
       HKEY_LOCAL_MACHINESystem<key name>Services<any service name>
       
       
    6. If you find such a key, delete it. For example, if you find an entry similar to:
       
       ImagePath = "C:WINNTwxyabc.exe"
       
       in the key:
       
       HKEY_LOCAL_MACHINESystemControlSet001Servicesxabc
       
       then delete the entire subkey "xabc."
       
       
13. Exit the Registry Editor.
    
    
14. Repeat steps c through e to change regedit.com back to regedit.exe.

5. To scan for and delete files detected as Backdoor.Exdis

1. On your Desktop, double-click My Computer.
2. Right-click your C: drive and select one of the following:
   • For Norton AntiVirus consumer products: "Scan with Norton Antivirus"
   • For Symantec AntiVirus Enterprise products: "Scan for Viruses..."
3. If any files are detected as infected with Backdoor.Exdis, click Delete.