VBS/Happytime is a VBS worm that propagates in two different ways - as a slow worm similar to JS/Kak, and as a fast worm - mass mailer.
Disinfection & Removal
Happytime first drops following files that contain the virus code:
help.hta help.htm help.vbs
Then it executes its payload, that activates if the sum of the day and the month is 13. At this time it deletes all files with extension ".dll" or ".exe".
Happytime.A uses a counter, and when it reaches number 366, then the worm sends itself replying to all messages listed in Outlook Inbox with a following message:
Subject: Fw: <email of the sender> Attachment: Untitled.htm
Subject: Help Attachment: Untitled.htm
where "Untitled.htm" is another file where the virus saves its code.
Next the worm replaces the current wallpaper with "Help.htm" via registry.
Happytime.A then prepares the system to send itself as a slow worm using Outlook Express 5.0. To do this, it creates a stationary that contains the worm code.
Finally the worm infects all files with ".htt" extension in the "\WEB" directory located in the Windows installation directory. Therefore the worm is executed each time when a folder viewed as a web page.
On the top of its code, the worm contains the following commented line:
I am sorry! happy time
Technical Details: Katrin Tocheva and Sami Rautiainen, F-Secure; May 2001