Threat behavior
Win32/Klez can take the following actions:
Drop the Elkern virus in locations such as %windir%, <temp folder>, or %ProgramFiles%.
Modify the Windows registry or a Windows system file so that the worm runs each time Windows starts.
Terminate processes, and delete files and registry keys associated with security-related applications and other malicious software.
Overwrite or corrupt files.
Depending on the variant, the worm can spread in several ways:
By copying itself or an archive of itself to mapped drives or remote network shares. The worm scans periodically for mapped drives and writeable shares.
By infecting executable files. The worm saves the original file with a random extension and then overwrites the original file with a worm copy. When an infected executable file starts to run, the worm runs unobtrusively and then runs the original uninfected executable so that no unusual behavior is readily apparent.
By sending a copy of itself as an e-mail attachment to addresses that it collects from Web sites or from the infected computer. The worm spoofs the e-mail sender address and randomly constructs the subject line, message body, attachment name, and attachment extension. The message body may indicate that the attachment is an infection-removal tool. The attachment name can have a double file extension, such as .txt.pif. The second extension can be hidden, which may make it appear that opening the attachment is safe. The worm runs when the user opens the attachment. If the computer does not have Security Bulletin MS01-020 or MS01-027 installed, the worm can exploit an Internet Explorer vulnerability in order to run automatically when the user previews or opens an e-mail that contains the attachment.
Prevention
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.
Use strong passwords.
Remove unneeded network shares.
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 in Windows XP
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 to have Automatic Updates notify you in step 5, 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. If you don't have antivirus software installed, you can get it from one of several companies. For more information, see http://www.microsoft.com/athome/security/downloads/default.mspx
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.
Use strong passwords
A strong password has at least eight characters and includes a combination of letters, numbers, and symbols. It is easy for you to remember, but difficult for others to guess. Weak passwords include any words in the dictionary, names, dates, consecutive letters or numbers, common words with symbol substitutions (for example, p@ssw0rd), and so on.
Remove unneeded network shares
Malicious software can often spread over network shares. Remove unneeded network shares that are mapped to your computer.
To remove network shares in Windows XP
On the Start menu, click My Computer.
On the Tools menu, click Disconnect Network Drives…
In the Disconnect Network Drives dialog box, click the drives to disconnect and click OK.
