VBS/Tam is a worm similar to JS/Kak. It uses the same security vulnerability to infect the system.
Microsoft has released a patch that fixes this vulnerability. It is available at http://www.microsoft.com/security/Bulletins/ms99-032.asp .
Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.
If an infected message is viewed, the worm creates a file, "tam.hta", to the startup directory of French version of Windows 9x ("C:\Windows\Menu demarrer\programmes\demarrage"). This file is executed when the system is restarted.
When the "tam.hta" is executed, it deletes file "c:\windows\out.html" if it exists. Then the worm creates a new file using the same file name. This file contains the worm code.
Next VBS/Tam.A checks if a file "out.hta" exists in the Windows directory, and if not, it copies the "tam.hta" there and hides "tam.hta".
The copied "out.hta" will be added to the registry, so it will be executed in each time when the system is restarted.
The worm replaces the signature settings of Outlook Express 5.0 with its own, so every email sent will contain the worm.
At August 30th, the it shows the following message four times:
Bon Anniversaire Lac !!! Un ami...
Depending on time user spends between the first and the last message box, the worm executes two different payloads.
The first one is activated if time is greater than 10 seconds, when the following message box is shown:
Ok, chante HappyBirthday tout ira bien!!!
Otherwise, VBS/Tam.A shows the following message
KOI??? Ca t'interresse pas? Tu n'es pas digne du monde informatique. BYE-BYE
and shuts down Windows.
Technical Details: Katrin Tocheva and Sami Rautiainen, F-Secure; October 2000