A malicious Trojan called "Skulls" is attacking Nokia's 7610 smart phone, and possibly other Symbian Series 60 devices, turning them into not-so-smart phones.

The software poses as a maintenance tool, but actually de-activates all links to Symbian system applications, such as e-mail and calendar, leaving the user able only to make and receive calls. The software replaces the menu icons with images of skulls.

"We have located several freeware and shareware sites offering a program, called Extended Theme Manager, that contains a Trojan horse," Mikko Hypponen, director of anti-virus research at Helsinki-based F-Secure, who issued a warning about Skulls on Friday. "The virus writer is going by the name Tee-222."

The Extended Theme Manager program looked "pretty convincing" as a freeware maintenance tool, said Hypponen. Many freeware sites have apparently not bothered to verify it or even try it out. Most monitored sites, he said, have since removed the program.

When installing the file "extended theme.sis", Symbian phone users are informed by the operating system (OS) that the software is not Symbian Signed - a trusted software application program initiated by the OS developer - and asked if they want to continue, according to Hypponen.

"This is definitely a good warning but the problem is that any advanced PC user who downloads software regularly sees this kind of warning 99 percent of the time and simply clicks OK," he said. "So the warning isn't really protecting much."

Most users should be able to pick themselves up and dust themselves off, as they can get the features back with a hard reset, said Hypponen. They will lose all private data, such as phone books and calendars, but will hopefully have a copy synchronised on their PC.

Earlier this year, the Symbian operating system software was the target of the Cabir virus, which, like Skulls, transmits a .sis file. But unlike Cabir, which scans for accessible phones within Bluetooth range and makes a copy of itself, Skulls is not self-replicating.