This follows Tuesday's warning that a number of variants of the Cabir worm have been detected, which have evolved beyond its comparatively benign predecessors. The worm affects phones running the Symbian operating system that use Bluetooth wireless technology.
Sophos said on Wednesday that a 32-year-old Brazilian programmer has published the source code for the Cabir-H and Cabir-I viruses online and said it could be used by virus writers to create their own variants. The programmer claims to have written the worms from scratch because so many people had contacted him asking if he had the source code for the original Cabir worm, according to Sophos.
Graham Cluley, senior technology consultant at Sophos, said users should be cautious about installing applications on their phone as more viruses may be written now the code is freely available.
"Publishing virus source code on the Web is dangerous because it encourages others to create malware," said Cluley in a statement. "Although viruses for mobile phones have to date been creating more hype than havoc, it's possible that more malicious people will now be investigating ways to infect cellphones. All users should be very careful about what applications they allow to install and run on their mobile device."
Symbian phone users can protect themselves by turning off the phone's "discoverable" mode, according to Sophos. It said it has not yet received a report from a member of the public infected by a mobile phone virus.
The malicious software affects only Symbian OS-based phones running Nokia's Series 60 user interface, according to Symbian.