W32.ExploreZip.L.Worm

SUMMARY

W32.ExploreZip.L.Worm is a variant of Worm.ExploreZip, a worm that contains a malicious payload. The file has been repacked to make it more difficult to detect with older, existing antivirus software. This worm is packed with the UPX file format, version 0.76.1-1.24.

The worm uses Microsoft Outlook, Outlook Express, or Exchange to mail itself, by replying to unread messages in the Inbox. The email attachment is titled Zipped_files.exe.

W32.ExploreZip.L.Worm also searches the mapped drives and network computers for Windows installations. If they are found, the worm copies itself to the Windows folder of the remote computer, and then modifies the Win.ini file of the infected computer.

Definitions dated from January 8, 2003 to January 10, 2003 will detect this worm as Worm.ExploreZip.

Protection

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

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 50 - 999
• Number of Sites: 3 - 9
• Geographical Distribution: Low
• Threat Containment: Moderate
• Removal: Moderate

Damage

• Damage Level: High
• Payload: In addition, when Worm.ExploreZip is executed, it searches through drives C through Z of your computer system and accessible network machines for the files. The worm selects a series of files to destroy multiple file extensions (including .h, .c, .cpp, .asm, .doc, .xls, .ppt), by calling CreateFile() and making them 0 bytes long. You may notice extended hard drive activity when this occurs. This can result in non-recoverable data. This payload routine continues while the worm is active on the system. Thus, any newly created files matching the extensions list will be destroyed as well.
• Large Scale E-mailing: Using MS Outlook/Express/MS Exchange
• Deletes Files: All files with the .c, .cpp, .h, .asm, .doc, .ppt, .xls extensions will be truncated to 0 bytes.
• Modifies Files: WIN.INI
• Degrades Performance: Increased hard-drive activity, unrecoverable loss of data

Distribution

• Distribution Level: High
• Subject of Email: Subject is the correspondent to a reply from a known email recipient on the previously sent email
• Name of Attachment: Zipped_files.exe
• Size of Attachment: One file: 91,048 bytes.
• Shared Drives: Infects all the local and mapped network drives
• Target of Infection: Windows 9x/NT systems

TECHNICAL DETAILS

W32.ExploreZip.L.Worm has a different file size than that of the original variant. However, the worm exhibits the same characteristics as the original Worm.ExploreZip worm. See the writeup, Worm.ExploreZip, for information on what this worm does.

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

To manually remove this worm, follow the steps that apply to your version of Windows:

• Windows 95/98/Me
  
  NOTE for Windows Me users only: Due to the file-protection process in Windows Me, a backup copy of the file you are to edit exists in the C:WindowsRecent folder. Symantec recommends that you delete this file before you continue with the steps in this section. To do this using Windows Explorer, go to C:WindowsRecent, and in the right pane select the Win.ini file, and then delete it. The file will be regenerated as a copy of the file you are to edit when you save your changes to that particular file.

1. Click Start, and then click Run.
2. Type the following:
   
   edit c:windowswin.ini
   
   and then click OK.
   
   (The MS-DOS Editor opens.)
   
   NOTE: If Windows is installed in a different location, make the appropriate path substitution.
   
3. In the [windows] section of the file, look for an entry similar to one of the following:
   
   run=?Explore.exe
   run=?\_setup.exe
   
4. If either of these entries exists, select the entire line. Make sure that you have not selected any other text, and then press Delete.
5. Restart the computer.
6. Open Windows Explorer.
7. Navigate to the C:WindowsSystem folder and, depending on which file name appeared in the run= line, delete Explore.exe or _setup.exe.


• Windows NT/2000/XP
  
  1. Start the Registry Editor (Regedit.exe).
  2. Navigate to the following key:
     
     HKEY_CURRENT_USERSoftwareMicrosoft
     WindowsNTCurrentVersionWindowsRun
     
     and remove the entry that refers to "Explore.exe" or "_setup.exe."
     
  3. Restart the computer, or kill the process using the Task Manager or Process View, if the file is currently in use.
  4. Depending on which file name appeared in the run line, delete Explore.exe or _setup.exe from the C:WinntSystem32 folder.

NOTE: For information on file recovery, see the Norton Utilities document, Error: "Cannot open file... if part of a ZIP format backup set..." and the Size of Many Files is Zero.