Mobile phones running Symbian's Series 60 operating system are the target of a new strain of the Skulls Trojan horse program. The new Trojan comes with the Cabir.B worm, which, unlike the first version of the virus, can spread to other phones within reach of Bluetooth broadcasting range.

"What is harmful about Skulls.B is that it can spread to other Bluetooth-enabled phones," said Mikko Hypponen, director of antivirus research at Helsinki-based F-Secure. "Skulls.A was bad in that it can wipe out all your applications, including your phone book, but it can't infect other phones."

Trojan horse programs are destructive and can modify the configuration of PCs but typically do not attempt to infect other machines, as do viruses and worms.

Although containing similar programming to its predecessor, Skulls.B doesn't replace the menu icons of Series 60 phones with images of skulls that render applications, such as e-mail and SMS (Short Message Service), useless. Instead, it uses Symbian default icons, which look like jigsaw puzzle pieces, but have the same destructive result as the skulls.

For users to infect their phones with Skulls.B, however, they have to make a bit of an effort: They need to press the Skulls.B icon in the menu to active the Trojan, according to Hypponen. A programming error prevents the virus from automatically running after installing itself on the phone, he said.

Discovered earlier this year, Cabir is a proof-of-concept worm that uses the Bluetooth protocol to copy itself onto devices around 30 feet away. It is transmitted as a Symbian installation system file and disguised as a security utility, called Caribe.

The Cabir worm drains a phone's battery relatively quickly because it is constantly trying to locate and connect with other Bluetooth-enabled devices, according to Hypponen.

F-Secure conducted tests on Series 60 smart phones from several vendors, including Nokia, Matsushita Electric Industrial (better known for its Panasonic brand), Sendo International and Siemens. All but one model, Siemens SX1, proved vulnerable, according to Hypponen. "I can't explain why the Siemens phone is immune to this virus but it is," he said.

F-Secure advises users of Series 60 smart phones to set their handsets into non-discoverable (hidden) Bluetooth mode and offers advice on fixing infected mobile phones on its Web site: www.f-secure.com