Worm.ExploreZip(pack)

SUMMARY

Worm.ExploreZip(pack) was discovered on Nov 24, 1999. This worm is a variant of Worm.ExploreZip, which was discovered in Israel, on Jun 6, 1999. The only difference is Worm.ExploreZip(pack) has been packed by file compression, which still allows the worm to be executed normally and does not require any manual uncompressing. Because of the compression, the file size is about 40% smaller than the original Worm.ExploreZip.

Threat Assessment

Wild

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

Damage

• Damage Level: Low
• Payload: The behavior of the worm is identical to Worm.ExploreZip. It can propagate itself very quickly through email automatically and also via the network. When the email is sent, it will have the following email content. I received your email and I shall send you a reply ASAP. Till then, take a look at the attached zipped docs. It will also contain a worm attachment named zipped_files.exe. In addition, when Worm.ExploreZip(pack) is executed, it searches drives C through Z of your computer system and accessible network machines for particular files. The worm then destroys all files containing any of the following extensions: .h, .c, .cpp, .asm, .doc, .xls, .ppt. This is accomplished by calling CreateFile(), and making the file extensions 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.

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

The worm utilizes MAPI-capable e-mail programs on Windows systems to propagate itself. The worm e-mails itself out as an attachment with the filename zipped_files.exe. The body of the e-mail message may appear to come from a known e-mail correspondent and contains the following text:

I received your email and I shall send
you a reply ASAP.
Till then, take a look at the attached
zipped docs.

Once the attachment is executed, it will unpack itself and execute the original Worm.ExploreZip routine. It may display an error message informing the user that the file is not a valid archive.

The worm proceeds to copy itself to the c:windowssystem directory with the filename explore.exe and then modifies the WIN.INI file so that the program is executed each time Windows is started. The worm then utilizes your e-mail client to harvest e-mail addresses in order to propagate itself. Users may notice that their e-mail client launches when this occurs.

The worm will continue to monitor the Inbox for new messages and respond accordingly.

The worm will also search the mapped drives and networked machines for Windows installations and copy itself to the Windows directory of the remote machine. It will then modify the WIN.INI accordingly.

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

Symantec AntiVirus Research Center has also provided a small utility named KILL_EZ to remove the virus from memory (in order to avoid rebooting from a clean system disk). Additional information regarding this tool may be found at the KILL_EZ Tool page.

1. Remove the line
   
   run=<Windows System Path>Explore.exe
   or
   run=<Windows System Path>\_setup.exe
   
   from the WIN.INI file for Windows 9x systems.
   
   For Windows NT, remove the registry entry
   
   HKEY_CURRENT_USERSoftwareMicrosoftWindows NTCurrentVersionWindowsRun
   
   which will refer to Explore.exe or _setup.exe
   
2. Delete the file Explore.exe or _setup.exe. One may need to reboot first or kill the process using Task Manager or Process View (if the file is currently in use).