Jail-breaking an iPhone handset invalidates the warranty says Apple.
A second worm to hit the iPhone has been unearthed by security company F-Secure.
It is specifically targeting people in the Netherlands who are using their iPhones for internet banking with Dutch online bank ING.
It redirects the bank's customers to a lookalike site with a log-in screen.
The worm attacks "jail-broken" phones - a modification which enables the user to run non-Apple approved software on their handset.
The handsets at risk also have SSH (secure shell) installed.
Many people use SSH so other programs can remotely connect to an iPhone and, among other things, transfer files. It comes with a default password, "alpine" which should be changed.
Only users who have installed SSH and not changed the password are at risk.
The new worm is more serious than the first because it can behave like a botnet, warns F-Secure.
This enables the phone to be accessed or controlled remotely without the permission of its owner.
"It's the second iPhone worm ever and the first that's clearly malicious - there's a clear financial motive behind it," F-Secure research director Mikko Hypponen told the BBC.
"It's fairly isolated and specific to Netherlands but it is capable of spreading."
He added although the number of infected phones was thought to be in the hundreds rather than thousands, the worm could jump from phone to phone among owners using the same wi-fi hotspot.
A spokesperson for ING Bank said that a warning was going to be put on the bank's official website.
"We are also briefing call centre personnel," she added. "It's important to remember that the worm only affects jail-broken phones and it is only aimed at customers in the Netherlands."
The first iPhone worm, called ikee, was harmless. Users with infected phones found their wallpaper replaced with a picture of 1980s popstar Rick Astley.
It also targeted jail-broken phones which were SSH enabled.
Its creator Ashley Towns said he wrote the ikee program in order to raise the issue of iPhone security.