The most famous mobile operating system to date is Android. It is used by millions of people worldwide and subsequently is the most targeted platform of all, by malicious users. A multitude of new viruses and pieces of malware or variants of existing ones is discovered every day by security companies, that vary – in relation to how much dangerous they are – from just popping up advertisements to stealing credentials and wiping a device completely. This thesis deals with a category of malware that charges users without their knowledge by sending SMS messages or initiating calls to premium rate numbers, known as premium-rate dialers and SMS fraudsters. In order to mitigate the risks stemming from this type of malicious programs, a custom mechanism is proposed that compels the user to perform some action before every attempt to send an SMS or place a call so that he is protected against any covert try to do so. The contribution of this master's thesis is a protective system that is composed of a random CAPTCHA puzzle that a user must solve correctly before the aforementioned attempts finish. Additionally, users can shift through the destination numbers of their actions and store them in databases so the next time they encounter any of these stored numbers, they are made aware that they are potentially dangerous or not.