This paper focuses on the behavior of Trojan horses in mobile devices. This malicious software tries to steal information from a mobile device while the user is unaware. We describe the communication links through a Trojan horse installed into a mobile device. To demonstrate the effects of a Trojan horse infection we present a practical example on a PDA. Via SMS, the malicious user can access a user’s contacts information through the previous installation of the Trojan horse. The results show that this process means a loss of information and a quantified cost to the attacked user too. This paper proposes different solutions to avoid this malware and its effects.