W32.Klez.gen@mm

W32.Klez.gen@mm is a mass-mailing worm that searches the Windows address book for email addresses and sends messages to all the recipients that it finds. The worm uses its own SMTP engine to send the messages.

The subject and attachment name of the incoming emails are randomly chosen. The attachment will have one of the extensions: .bat, .exe, .pif, or .scr.

The worm exploits a vulnerability in Microsoft Outlook and Outlook Express to try execute itself when you open or preview the message. Information and a patch for the vulnerability can be found at:

http://www.microsoft.com/technet/security/bulletin/MS01-020.asp.

W32.Klez.gen@mm attempts to copy itself to all the network-shared drives that it finds.

Depending on the variant of the worm, it will drop one of the following viruses:

• W32.Elkern.3326
• W32.Elkern.3587
• W32.Elkern.4926

which then infects the system.

Email spoofing
Some variants of this worm use a technique known as "spoofing" by which the worm randomly selects an address it finds on an infected computer. The worm uses this address as the "From" address when it performs its mass-mailing routine. Numerous cases have been reported in which users of uninfected computers received complaints that they sent an infected message to another individual.

For example, Linda Anderson is using a computer infected with W32.Klez.E@mm. Linda is neither using an antivirus program nor has the current virus definitions. When W32.Klez.gen@mm performs its email routine, it finds the email address of Harold Logan. The worm inserts Harold's email address into the "From" portion of an infected message, which it then sends to Janet Bishop. Then, Janet contacts Harold and complains that he sent her an infected message; however, when Harold scans his computer, Norton AntiVirus does not find anything, because his computer is not infected.

If you are using a current version of Norton AntiVirus and you have the most recent virus definitions, and a full system scan with Norton AntiVirus, which is set to scan all the files, does not find anything, your computer is not infected with this worm.

Recommendations

Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

• Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
• 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.
• Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
• Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
• Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
• Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
• If a 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.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate compromised computers quickly to prevent threats from spreading further. 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.
• If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
• For further information on the terms used in this document, please refer to the Security Response glossary.

Summary