Malicious Software Encyclopedia: Win32/Kelvir

Published: September 5, 2006

Win32/Kelvir is a family of worms that targets computers running certain versions of Microsoft Windows.

The worm spreads through MSN Messenger or Windows Messenger. The worm is activated when a user clicks a link in a Messenger message.

Severity

What the levels mean

Glossary Terms

Click the term to get the definition from our Security Glossary.

•	Trojan horse
•	virus
•	worm

Threat Overview

Class/type	Worm - Program-specific
Discovered	April 6, 2005
Circulating	Yes
Affected operating systems	Windows 95 Windows 2000 Windows XP Windows Server 2003
Affected software	MSN Messenger
Infection rating	Low
Recovery difficulty	Easy
Damage rating	Low
Transmission rating	Medium

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Aliases (Also Known As)

Different antivirus vendors may be using different names to refer to this malicious software. Here are some of the names currently in use by antivirus software vendors participating in the Microsoft Virus Information Alliance (VIA):

McAfee: W32/Kelvir.worm.gen

Sophos: W32/Kelvir-Gen

Learn more about the Microsoft Virus Information Alliance.

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Technical Analysis

When Win32/Kelvir runs, it takes the following actions:

Starts MSN Messenger or Windows Messenger in the background, if the program is not already running.
Sends a message to all contacts in the user's MSN Messenger or Windows Messenger contacts list. The message prompts the recipient to click a link in the message. Clicking this link installs and runs Win32/Kelvir on the user's computer.
Downloads a file from a specific Web site and runs the file on the computer. The downloaded file may be a variant of the Backdoor:Win32/Rbot Trojan.

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How to Prevent Infection

Take the following steps to help prevent infection on your system:

Enable a firewall on your computer.
Get the latest computer updates.
Use up-to-date antivirus software.
Use caution with unknown attachments.

Enable a firewall on your computer

Use a third-party firewall product or turn on the Microsoft Windows XP Internet Connection Firewall.
To turn on the Internet Connection Firewall in Windows XP

Click Start, and click Control Panel.
Click Network and Internet Connections, and click Network Connections. If you do not see Network and Internet Connections, click Switch to Category View.
Highlight a connection that you want to help protect, and click Change settings of this connection.
Click Advanced, and select Protect my computer and network by limiting or preventing access to this computer from the Internet.
Click OK.

Get the latest computer updates

Updates help protect your computer from viruses, worms, and other threats as they are discovered. You can use the Automatic Updates feature in Microsoft Windows XP to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
To turn on Automatic Updates

Click Start, and click Control Panel.
Click Performance and Maintenance. If you do not see Performance and Maintenance, click Switch to Category View.
Click System.
Click Automatic Updates, and select Keep my computer up to date.
Select a setting. Microsoft recommends selecting Automatically download the updates, and install them on the schedule that I specify and setting a regular update time.
If you choose in step 5 to have Automatic Updates notify you, you will see a notification balloon when new downloads are available to install. Click the notification balloon to review and install updates.

Use up-to-date antivirus software

Most antivirus software can detect and prevent infection by known malicious software. You should always run antivirus software on your computer that is updated with the latest signature files to automatically help protect you from infection.

Use caution with unknown attachments

Use caution before opening unknown e-mail or IM attachments, even if you know the sender. If you cannot confirm with the sender that a message is valid and that an attachment is safe, delete the message immediately, and run up-to-date antivirus software to check your computer for viruses.

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