W32.Yaha.F@mm is a mass-mailing worm that sends itself to all email addresses that exist in the Microsoft Windows Address Book, the MSN Messenger List, the Yahoo Pager list, the ICQ list, and files that have extensions that contain the letters ht. The worm randomly chooses the subject and body of the email message. The attachment will have a .bat, .pif or .scr file extension. Depending upon the name of the Recycled folder, the worm either copies itself to that folder or to the %Windows% folder.
The name of the file that the worm creates consists of four randomly generated characters between c and y.
It also attempts to terminate antivirus and firewall processes.