Trojan.Gutta

SUMMARY

Trojan.Gutta is a Trojan horse that disables access to .exe files.

Protection

• Virus Definitions (LiveUpdate™ Weekly) February 11, 2004
• Virus Definitions (Intelligent Updater) February 10, 2004

Threat Assessment

Wild

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

Damage

• Damage Level: Medium
• Payload: Disables access to exe files.
• Degrades Performance: Disables access to executables.

Distribution

• Distribution Level: Low
• Subject of Email: no mail

TECHNICAL DETAILS

Trojan.Gutta disables access to files with the .exe extension. When this Trojan runs, it performs the following actions:

1. Copies itself as C:WindowsCSRSS.exe. This path is hard-coded and does not depend on system variables.
   
   
2. Creates the value:
   
   "rundll32" = "windowscsrss.exe"
   
   in the registry key:
   
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRun
   
   so that the Trojan attempts to start when you start Windows.
   
   
3. Modifies the value of:
   
   (Default)
   
   in the registry key:
   
   HKEY_CLASSES_ROOTexefileshellopencommand
   
   by adding:
   
   hahafool
   
   to the Value data. As a result, when you try to run a .exe file, it will not open. For example, if you try to run Notepad.exe after the modification is made, instead of looking for Notepad.exe, it looks for Notepad.exehahafool.
   
   

This Trojan horse does not have backdoor or worm functionality. Because it modifies access to .exe files, it cannot run when Windows is restarted, but will generate an error message.

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. Reverse the changes that were made to the registry.
2. Disable System Restore (Windows Me/XP).
3. Update the virus definitions.
4. Run a full system scan and delete all the files detected as Trojan.Gutta.
   

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

1. Reversing the changes that were made to the registry.
Because the worm modified the registry so that you cannot run any .exe files, first make a copy of the Registry Editor as a file with the .com extension, and then run that file.

1. Do one of the following, depending on the version of Windows you are running:
   • Windows 95/98 users:
     1. Click Start.
     2. Point to Programs.
     3. Click the MS-DOS Prompt. (A DOS window opens at the C:Windows prompt.) Proceed to step B of this section.
        
        
   • Windows Me users:
     1. Click Start.
     2. Point to Programs.
     3. Point to Accessories.
     4. Click the MS-DOS Prompt. (A DOS window opens at the C:Windows prompt.) Proceed to step B of this section.
        
        
   • Windows NT/2000 users:
     1. Click Start, and then click Run.
     2. Type command, and then press Enter. (A DOS window opens.)
     3. Type cd winnt, and then press Enter.
     4. Go to step B of this section.
        
        
   • Windows XP users:
     1. Click Start, and then click Run.
     2. Type command, and then press Enter. (A DOS window opens.)
     3. Type the following:
        
        cd
        cd windows
        
        Press Enter after typing each one.
        
        
     4. Proceed to step B of this section.
        
        
2. Type copy regedit.exe regedit.com
   
   and then press Enter.
   
   
3. Type start regedit.com
   
   and then press Enter. (The Registry Editor opens in front of the DOS window.)
   
   After you finish editing the registry, exit the Registry Editor, and then exit the DOS window as well.
   
   
4. Before continuing, 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. For instructions, read the document, "How to make a backup of the Windows registry."
   
   
5. Navigate to and select the key:
   
   HKEY_CLASSES_ROOTexefileshellopencommand
   
   NOTE: The HKEY_CLASSES_ROOT key contains many subkey entries that refer to other file extensions. One of these file extensions is .exe. Changing this extension can prevent any files ending with a .exe extension from running. Make sure that you completely browse through this path until you reach the command subkey.
   
   Modify the HKEY_CLASSES_ROOTexefileshellopencommand subkey, shown in the following figure:
   
   <<=== NOTE: Modify this key.
   
   
6. In the right pane, double-click the (Default) value.
7. Click in the Value data field to the right of the text so that the current value data is no longer selected in blue.
8. Backspace to delete the characters:
   
   hahafool
   
   Make sure that you do not delete the text:
   
   "%1" %*
   
   
9. Click OK.
   
   
10. Navigate to the key:
    
    HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionRun
    
    
11. In the right pane, delete the value:
    
    "rundll32"="C:WindowsCSRSS.exe"
    
    
12. Exit the Registry Editor.
    

2. 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.

3. 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.

4. 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 any files are detected as infected with Trojan.Gutta, click Delete.