Worm.ExploreZip

On January 8, 2003, Security Response discovered a packed variant of this threat which exhibits the same characteristics. Protection will be available for this new variant in virus definitions dated 1/8/2003 with a version number of 50108q (20030108.017) or greater.

Worm.ExploreZip is a worm that contains a malicious payload. The worm utilizes Microsoft Outlook, Outlook Express, or Exchange to mail itself out by replying to unread messages in your Inbox. The email attachment is Zipped_files.exe.

The worm also searches mapped drives and networked computers for Windows installations. If found, it copies itself to the \Windows folder of the remote computer and then modifies the Win.ini file of the infected computer.

Also Known As:	W32.ExploreZip Worm, I-Worm.ZippedFiles [AVP], Win32/ExploreZip.Worm [CA], W32/ExploreZip@MM [McAfee]
Type:	Worm
Infection Length:	210,432 bytes
Systems Affected:	Windows 95, Windows 98, Windows NT, Windows 2000, Windows XP, Windows Me
Systems Not Affected:	Windows 3.x, Macintosh, UNIX, Linux

Virus Definitions (LiveUpdate™ Weekly) June 09, 1999 Virus Definitions (Intelligent Updater) June 09, 1999

Wild Number of infections: 50 - 999 Number of sites: 3 - 9 Geographical distribution: Low Threat containment: Moderate Removal: Moderate

Threat Metrics Wild: Low Damage: High Distribution: High

Damage

• Payload: In addition, when Worm.ExploreZip is executed, it also searches through the C through Z drives of your computer system and accessible network machines for particular files. The worm selects a series of files to destroy of multiple file extensions (including .h, .c, .cpp, .asm, .doc, .xls, .ppt) by calling CreateFile() and making them 0 bytes long. One may notice extended hard drive activity when this occurs. This can result in non-recoverable data. This payload routine continues to happen 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 .c, .cpp, .h, .asm, .doc, .ppt, .xls extensions
  • Modifies files: WIN.INI
  • Degrades performance: Increased hard-drive activity, unrecoverable loss of data

Distribution

• Subject of email: Subject is correspondet to a reply from a known e-mail recipient on the previously sent e-mail
• Name of attachment: Zipped_files.exe
• Size of attachment: One file
• Shared drives: Infects all local and mapped network drives
• Target of infection: Windows 9x / NT systems

The payload of the worm destroys any file with the extension .h, .c, .cpp, .asm, .doc, .ppt, or .xls on the hard disk of the infected computer, any mapped drives, and any network computers that are accessible when it is executed. This continues to occur until the worm is removed.

You may receive the worm as an attachment named Zipped_files.exe . When run, this executable copies itself to the \Windows\System folder with the file name Explore.exe or to the \Windows folder with the file name _setup.exe. The worm modifies Win.ini or the registry so that Explore.exe is executed each time you start Windows.

The worm was first discovered in Israel and submitted to Symantec Security Response on June 6, 1999.

Worm.ExploreZip uses MAPI commands and Microsoft Outlook, Outlook Express, or Exchange on Windows 95/98/NT to propagate itself.

The worm emails itself as an attachment with the file name Zipped_files.exe in reply to unread messages in your inbox. Once the worm responds to a message in your inbox, it marks the message so it will not respond to the message again. Because of this, the messages appear to the recipients to be replies to their messages. The message contains the following text:

Hi <Recipient Name>!
I received your email and
I shall send you a reply
ASAP. Till then, take a
look at the attached
zipped docs.

bye,

<your name>

or

sincerely,

<your name>

The worm continues to monitor the inbox for new messages and responds accordingly.

Once the attachment is executed, the following message may appear:



The displayed button is the "OK" button and is dependent on the language of the infected operating system. The example shown was taken from a Hebrew language \Windows.

The worm copies itself to the \Windows\System (\System32 on Windows NT) folder with the file name Explore.exe or _setup.exe and also modifies the Win.ini file (Windows 95/98) or the registry (Windows NT) so that the program is executed each time that Windows is started. You may also find this file in the \Windows temporary folder, or the attachments folder, depending on the email program you are using.

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.

Symantec Security Response has provided a small utility named FixExzip.exe to remove the virus from memory and undo the changes made by the worm. Please click here to obtain this tool. This is the recommended method for removing this worm.

If you want to remove this worm manually, follow these steps:

• If you are running Windows 95/98:
  1. Open the Win.ini file in a text editor such as the System Configuration Editor (Sysedit.exe).
  2. Remove the command C:\Windows\System\Explore.exe or C:\Windows\System\_Setup.exe from the run= line.
  3. Restart the computer.
  4. Delete Explore.exe from the C:\Windows\System folder.
     
• If you are running Windows NT:
  1. Start the Registry Editor (Regedit.exe).
  2. Navigate to the following key:
     
     HKEY_CURRENT_USER\Software\Microsoft\WindowsNT\CurrentVersion\
     Windows\Run
     
     and remove the entry that refers to "Explore.exe" or _setup.exe."
     
  3. Restart the computer, or kill the process using Task Manager or Process View (if the file is currently in use.)
  4. Delete Explore.exe from the C:\Winnt\System32 folder.

Revision History:

• July 1, 2003: Added link to updated removal tool.
• January 8, 2003. Security Response discovered a variant which contains the same characteristics previously described but has been packed by UPX. This version is 91,408 bytes in size. Virus Definitions dated 1/8/2003 with a version number of 50108q or greater will detect this variant.
• September 20, 2002. Downgraded from Category 3 to Category 2 based on decreased rate of submissions.

Write-up by: Eric Chien