Skulls Trojan attacks Symbian mobile phones

Users of Nokia's 7610 smart phone and possibly other phones running Symbian's Series 60 software should be aware of a new Trojan program on the Internet.

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"We have located several freeware and shareware sites offering a program, called Extended Theme Manager, that contains a Trojan horse," Mikko Hyppönen, director of antivirus research at Helsinki-based F-Secure, said Monday in an interview. "The virus writer is going by the name Tee-222."

The malicious code, called Skulls, deactivates all links to Symbian system applications, such as e-mail and calendar, by replacing their menu icons with images of skulls, according to Hyppönen. Users of affected phones can only send or receive calls, he said.

F-Secure issued a warning on Friday.

Hyppönen said that the Extended Theme Manager program looked "pretty convincing" as a freeware maintenance tool and that many sites had 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 Hyppönen.

"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."

One way to correct the problem, Hyppönen said, is a hard reset, which restores affected phones to their default factory setting. Unfortunately, all private data, such as phone books and calendars, is lost in the process.

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.