Smartphone malware danger warning from experts
7 November 2011 Last updated at 04:06 GMT
Experts are warning that criminals are creating fake apps that secretly send premium rate text messages from smartphones without the user knowing.
The apps, which can be installed without the user realising they are trojans, can lead to massive bills.
Rik Ferguson, an "ethical hacker" and expert on smartphone security, demonstrates how easy it is for criminals to create trojan apps.