Welyah is a mass-mailer worm that is 110 kilobytes in size and was written in Visual Basic.
The worm utilises it's own SMTP engine so it does not depend on Outlook for e-mail sending. The recipient addresses are collected from different files in the system, e.g: *.wab files (Windows Address Book), *.mbx (Mailbox).
Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.
Messages sent by Welyah look like this:
The attachment name is disguised as a text file but it has .pif extension that is not visible because of the many space characters before it.
The worm also uses the IFRAME vulnerabilty that makes Outlook to execute the attachment automatically. More information on the vulnerabilty can be found athttp://www.microsoft.com/windows/ie/downloads/critical/q323759ie/default.asp
When the attachment in executed it copies itself to the Windows directory as 'Winl0g0n.exe' and adds it to the runkeys in the registry as
so the worm will be started when Windows starts up.
Technical Details: Gergely Erdelyi; F-Secure Corp.; 20th of December, 2001