The Beselo.A and Beselo.B worms are in the wild, looking to lure Symbian S60 users into clicking on their incoming malicious files, according to a warning issued on Tuesday by F-Secure.

The Beselo worms are tricky, in that they use common media file extensions, rather than a standard SIS extension, in sending their malicious payload.

Like the Commwarrior worms, the Beselo worms rely on MMS and Bluetooth to get around, with some social engineering thrown in to trick users into installing the SIS application installation file. But because this file has a common media file extension, such as beauty.jpg, sex.mp3, or love.rm, users are more likely to click "yes" to an installation prompt when opening the file, notes F-Secure.

F-Secure offers this word of advice: just say "no" to such a request.

"There is no reason for any image file to ask installation questions on the Symbian platform, so any image or sound file that does something else than play immediately is without question something else than it claims to be," warns F-Secure.

That's the latest twist on smart-phone worms, which debuted in 2004 with the arrival of the Cabir worm. The Beselo worms, meanwhile, were initially clumped in with the pervasive Commwarrior worms, until a discovery was made about their use of common media file extensions.