An Australian hacker claims to have written the first iPhone worm, though only for phones that have been "jailbroken".
The 21-year-old hacker, Ashley Towns, has written a worm - a type of self-replicating computer program similar to a virus, that changes the iPhone's wallpaper to show a picture of 80's pop singer Rick Astley and displays the message "ikee is never going to give you up".
Mr Towns said he had created the virus to make users aware of the danger of not changing the default password for their phone.
However, only iPhone users who have 'jailbroken' their phones will be affected by the worm. Jailbreaking an iPhone involves running a program that circumvents the official Apple operating system and allows users to run software on their phone that has not been approved by Apple.
Apple doesn't support jailbroken phones and has tried to prevent jailbreaking through software updates. The company has also claimed that iPhone jailbreaking is illegal.
Writing on his blog, Mikko Hypponen of security company F-Secure said that source code had been released for four variants of the worm. Mr Hypponen wrote: "This means that there will quickly be more variants, and they might have nastier payload than just changing your wallpaper."
Users who have jailbroken their phones are vulnerable to Mr Towns's worm and those similar to it only if they are running a program called SSH, which allows people to connect to the phone remotely over the internet. Even then, users can remove the threat from the virus by changing their password from the default, which is "alpine".
Last week a Dutch hacker began accessing vulnerable phones and demanding money for instructions on how to fix the loophole.