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.
Users who have installed SSH and not changed the password are especially 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.