However, the worm, known as Ikee, is only a threat to users who have jaibroken their phones to let them run unauthorized software, security experts say.

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In fact, Ikee doesn't do anything particularly bad -- it changes the victim's wallpaper to a photograph of 80s singer Rick Astley and then seeks out other phones to infect -- but it could be modified to do something more dangerous such as stealing sensitive information from the iPhone. "There is a real danger that someone could take this code and make it do something malicious," said Graham Cluley, a technology consultant with security vendor Sophos.

The worm does not affect most iPhone users; only those with jailbroken iPhones that are running a Unix utility called SSH (Secure Shell) with the iPhone's default password, "alpine," still in use. SSH lets someone connect to the iPhone remotely over the Internet, so installing this software with the default password in place is akin to adding an unlocked back-door to the device. It doesn't affect users who use the phone in conjunction with Network Address Translation (NAT), a popular networking technology that lets many users share the same IP address.

Security experts have known about this particular risk for some time now. Last week a Dutch hacker started hacking into iPhones that were vulnerable to this attack and demanded €5 (US$7.43) for instructions on how to fix the problem.

The worm was written Wednesday by Ashley Towns, a 21 year-old unemployed programmer from Wollogong, Australia. He hadn't heard of the Dutch incident when he released his worm, he said Sunday in an instant-message interview. "It was supposed to be a small prank i definitely wasn't expecting it to get as far as it did," he said.

Towns wanted to make the point that people should change their default iPhone passwords, especially if they're using SSH . "A lot of people especially at first thanked me," he said. "I think most people are relieved its not out to destroy their phone. I have had a few people abuse me though."

He didn't really consider the possible legal ramifications of releasing the code, something he said was "quite naive of me," Towns said.

Once the worm adds the Rick Astley wallpaper, it disables the iPhone's SSH daemon and then looks around and then looks around on the Internet for other vulnerable iPhones to infect.