The world's first iPhone virus is reported to be in the wild, targeting users who have cracked, or “jailbroken”, their phone.

The virus, which so far appears to be confined to Australia, changes the user’s wallpaper to an image of 1980s pop star Rick Astley, and displays the message “Ikee is never gonna give you up”.

Only those users who have cracked, or jailbroken, their phones and not changed their passwords from the default – "alpine" – can be affected.

Security firm Sophos said that once in place, the worm attempts to find other iPhones on the mobile phone network that are similarly vulnerable, and installs itself on them.

"This is a wake-up call to iPhone users around the world to take greater care about their security - especially if they jailbreak their phones," said Sophos senior technology consultant, Graham Cluley.

"Businesses also need to ensure that they don't have staff who are endangering corporate data by running insecure smartphones. Other inquisitive hackers may also be tempted to experiment, and could take the code of ikee and adapt it to have a more sinister payload."

Sophos researchers have traced the virus back to Ashley Towns, 21-year-old student from New South Wales. Towns admitted to releasing the worm on his Twitter page, saying that he did so because he found that 26 out of 27 accessible iPhones he had tested were vulnerable because they had not changed their passwords.

Finnish security firm F-Secure has also warned about the virus, and advised iPhone users to expect further attacks.

"The creator of the worm has released full source code of the four existing variants of this worm. This means that there will quickly be more variants, and they might have nastier payload than just changing your wallpaper or might try password cracking to gain access to devices where the default password has been changed," the firm said in a blog post.