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Virus Encyclopedia

Welcome to the Virus Encyclopedia of Panda Security.

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Threat LevelHigh threatDamageSevereDistributionNot widespread
Common name:Klez.F
Technical name:W32/Klez.F
Threat level:Medium

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

Affected platforms:

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

First detected on:
Detection updated on:Oct. 31, 2007
Proactive protection:
Yes, using TruPrevent Technologies

Brief Description 


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.

Visible Symptoms 


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.