It drops the Elkern.A virus, prevents the computer from starting up correctly and renders infected files unusable.

Windows XP/2000/NT/ME/98/95

Klez.F is a worm that reaches computers in an e-mail message.

It is very easy to become infected by this worm, as it is automatically activated when the message is viewed through Outlook’s Preview Pane. It does this by exploiting a vulnerability in Internet Explorer (versions 5.01 and 5.5), which allows files attached to e-mail messages to be automatically run.

The dangers of Klez.F are the following:

• It automatically and rapidly spreads to all the contacts in Outlook’s Address Book.
• It drops the Elkern.A virus in the affected computer, which prevents it from starting up correctly and renders certain programs unusable.

Klez.F is difficult to recognize because it is hidden in e-mail messages with variable characteristics. The most common subjects are:

• A powful tool
• Worm Klez.E immunity
• A  funny website.

For more information consult the section Means of transmission.