Threat Description



Category: Malware
Platform: W32
Aliases: Kak, Wscript.KakWorm, KakWorm


Kak is a worm that - like BubbleBoy - embeds itself without any attachment to every e-mail sent from the infected system. For further information about BubbleBoy, see the description:


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.

More scanning & removal options

More information on scanning or removal options is available in the documentation for your F-Secure security product on the Downloads section of our Home - Global site.

You may also refer to the Knowledge Base on the F-Secure Community site for more information.

Contact Support

For further assistance, F-Secure customers can request support online via the Request support or the Chat forms on our Home - Global site.

Technical Details

Kak is written in JavaScript and it works on both English and French versions of Windows 95/98 if Outlook Express 5.0 is installed. It does not work in a typical Windows NT installation.

The worm uses a known security vulnerability that is in Outlook Express. Once the user receives an infected e-mail message and opens or views the message in the preview pane, the worm creates a file "kak.hta" to the Windows Startup directory.

Next time the system is restarted, the worm activates. It replaces "c:\autoexec.bat" with a batch file that deletes the worm from the Startup directory. The original "autoexec.bat" is copied to "C:\AE.KAK".

Also, It modifies the message signature settings of Outlook Express 5.0 by replacing the current signature with an infected file, "C:\Windows\kak.htm".

Therefore every message sent with Outlook Express will contain the worm after this has been done.

Then it modifies the Windows registry in such a way that it will be executed in every system startup. The key it adds to the registry is:


The .hta file that the virus creates and executes in the future is saved to Windows System directory. On the first day of each month, if the number of hours is more than 17 (i.e. 6pm or later), the worm will show an alert box with the following text:

    Kagou-Anit-Kro$oft say not today!  

Then the worm shuts down Windows.

F-Secure Anti-Virus detects the worm. When the worm has been detected, the user should delete the following files, if they exist:

    C:\Windows\kak.htm     C:\Windows\System\(filename).hta 	 	

where (filename) is a variable, and it changes from one system to another

    C:\Windows\Start Menu\Programs\Startup\kak.hta     C:\Windows\Menu Demarrer\Programmes\Demarrage\kak.hta 	 	

The "autoexec.bat" file can be restored by renaming "C:\AE.KAK" to "C:\autoexec.bat".

Kak uses a known security hole in Microsoft Outlook Express to create the local HTA file.

If active scripting is disabled from Outlook Express, the worm will not work.

Microsoft has more information on this problem available at:

They also have a patch to fix this problem at:

Technical Details:Katrin Tocheva and Sami Rautiainen, F-Secure


Suspect a file or URL was wrongly detected? Submit a sample to our Labs for analysis

Submit Now

Give And Get Advice

Give advice. Get advice. Share the knowledge on our free discussion forum.

Learn More