W32.Zellome@m

SUMMARY

W32.Zellome@m is a worm that uses it own SMTP engine to send an email to addresses gathered from a compromised computer.

Protection

• Virus Definitions (LiveUpdate™ Weekly) February 27, 2005
• Virus Definitions (Intelligent Updater) February 25, 2005

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
• Large Scale E-mailing: Sends a single email to multiple recipients
• Deletes Files: Worm might attempt to rename or delete files using randomly chosen names, which might match some existing files
• Modifies Files: Worm might create directories with random names

Distribution

• Distribution Level: Medium
• Subject of Email: juas juas cuidadin con el attachhhhrrrr!!!!!
• Name of Attachment: soyunpeasodebichooooooo.scr
• Size of Attachment: Approximately 70kb-2Mb

TECHNICAL DETAILS

When W32.Zellome@m runs, it does the following:

1. Copies itself to the %System% directory as "bigfish.scr".
   
   Note: %System% is a variable that refers to the System folder. By default this is C:WindowsSystem (Windows 95/98/Me), C:WinntSystem32 (Windows NT/2000), or C:WindowsSystem32 (Windows XP).
   
   
2. Adds the value:
   
   "(Default)" = "%System%bigfish.scr"
   
   to the registry subkey:
   
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionRunigfish
   
   so that the worm runs every time Windows starts. This modification only works on Windows 2000 systems. Other operating systems would not process this key.
   
   
3. If the email attachment received contains a copy of the polymorphic engine used by the worm, the following file is created:
   
   %Windir%incubator.scr
   
   Note: %Windir% is a variable that refers to the Windows installation folder. By default, this is C:Windows or C:Winnt.
   
   
4. Adds the value:
   
   "(Default)" = "%Windir%incubator.scr"
   
   to the registry subkey:
   
   HKEY_CURRENT_USERSOFTWAREMicrosoftWindowsCurrentVersionRunincubator
   
   so that the worm runs every time Windows starts. This modification only works on Windows 2000 systems. Other operating systems would not process this key.
   
   
5. Queries the following registry key to find the default mail account. It will query the "Accounts" key for the "SMTP Server" value. This value is the name of the SMTP server that will be used by W32.Zellome@m to send itself:
   
   HKEY_LOCAL_MACHINESoftwareMicrosoftInternet Account ManagerDefault Mail Account
   
   
6. May copy the polymorphic engine to the %Windir% directory as "incubator.scr", if it arrived with the attachment.
   
   
7. Adds the value:
   
   "debugger" = "%Windir%incubator.scr"
   
   to the registry subkeys:
   
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Optionsscrnsave.scr
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Optionsegedt32.exe
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Optionsegedit.exe
   
   so that attempting to run any of these files will cause W32.Zellome@m to run instead, and the real files cannot be executed anymore. This applies to Windows NT based operating systems only. This will not be processed on a Windows 9x based system.
   
   
8. Adds the value:
   
   "debugger" = "%System%igfish.scr"
   
   to the registry key:
   
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Options askmgr.exe
   
   so that attempting to run the Windows Task Manager will cause W32.Zellome@m to run instead. The real taskmgr.exe cannot be executed anymore. This applies to Windows NT based operating systems only. This will not be processed on a Windows 9x based system.
   
   
9. Uses a polymorphic engine (if present) to produce a replicant with a different appearance, which will also send itself through email. The polymorphic engine produces highly obfuscated C++ source code which is then compiled by W32.Zellome@m to produce the replicants. This technique is similar in idea to that of W32.Apparition, yet much more advanced.
   
   
10. Uses it own SMTP engine to send an email to addresses gathered from the Windows Address Book and up to 40 addresses found in the the browser cache directory in the following file extensions:
    
    
    • .htm
    • .asp
    • .xml
      
      
11. The email characteristics are constant, except for the file size, which is highly variable and approximately 70kb-2Mb. The email has the following characteristics:
    
    From:
    Don Quijote y Sancho Panza
    
    Subject:
    juas juas cuidadin con el attachhhhrrrr!!!!!
    
    Message Body:
    juas juas juas peaso de bicho que lleva el attach!!! juas juas!!! ;D
    Vallez29a
    
    Attachment:
    soyunpeasodebichooooooo.scr
    
    
12. May contain a copy of the polymorphic engine in the attachment.
    
    
13. May be a UPX packed attachment.

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 W32.Zellome@m.
4. Restart in Safe Mode.
5. Restore regedit.exe from your original Windows installation disk.
6. Delete the value that was added to the registry.
   

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

1. To disable 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"

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.

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. To update 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 daily. 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. To scan for and delete 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 W32.Zellome@m, click Delete.
   
   Note: If your Symantec antivirus product reports that it cannot delete an infected file, Windows may be using the file. To fix this, run the scan in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode." Once you have restarted in Safe mode, run the scan again.
   
   When all the infected files have been deleted, restart the computer in Normal mode.
   
   
   

4. To restart the computer in Safe mode or Safe mode with Command Prompt
Follow the instructions for your operating system.

Windows 95/98/Me
Shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."

Once in Safe mode (this could take some time) proceed with section 4.

Windows 2000

1. Shut down the computer, and then turn off the power. Wait for at least 30 seconds, and then restart the computer
2. When you see the black and white Starting Windows bar at the bottom of the screen, press the F8 key (usually on the top row of the keyboard).
3. In the Windows 2000 Advanced Options Menu, select Safe mode with Command Prompt, and then press Enter.
   
   Once the computer opens to a window with a command prompt (you should see a line of text and a blinking cursor), proceed with section 4.

Windows XP

1. Shut down the computer, and then turn off the power. Wait for at least 30 seconds, and then restart the computer. The computer begins processing a set of instructions known as the Basic Input/Output System (BIOS). What is displayed depends on the BIOS manufacturer. Some computers display a progress bar that refers to the word BIOS, while others may not display any indication that this process is occurring.
2. As soon as the BIOS has finished loading, begin tapping the F8 key on your keyboard. Continue to do so until the Windows Advanced Options menu appears. If you begin tapping the F8 key too soon, some computers will display a "keyboard error" message. To avoid this, restart the computer and try again.
3. In the Windows 2000 Advanced Options Menu, select Safe mode with Command Prompt, and then press Enter.
   
   Once the computer opens to a window with a command prompt (you should see a line of text and a blinking cursor), proceed with section 4.

5. Restore regedit.exe from your original Windows installation disk.

1. Insert your Windows Installation CD into your CD-ROM drive.
2. Navigate to the drive corresponding to your CD-ROM drive (e.g. if your CD-ROM uses drive letter e: you would type e:
3. Regedit.exe needs to be renamed in order to run. To copy regedit onto your system and rename it type:
   copy I386egedit.exe c:Windowssymfix.exe .
   
4. Navigate back to your c: (type c: )
5. Type Windowssymfix.exe to open and run the copied version of regedit.
   
   

6. To delete the value from the registry
Important: 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 subkeys only. Read the document, "How to make a backup of the Windows registry," for instructions.

After you finish editing the registry, exit the Registry Editor, and then exit then restart your computer.

1. Navigate to the subkey:
   
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionRun
   
   
2. In the left pane, delete the subkey:
   
   bigfish
   
   
3. Navigate to the subkey:
   
   HKEY_CURRENT_USERSOFTWAREMicrosoftWindowsCurrentVersionRun
   
   
4. In the left pane, delete the subkey:
   
   incubator
   
   
5. Navigate to the following subkeys and delete them:
   
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Options askmgr.exe
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Optionsscrnsave.scr
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Optionsegedt32.exe
   HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows NTCurrentVersionImage File Execution Optionsegedit.exe
   
   
6. Exit the Registry Editor.