Abstract
The abuse of free web-based services has been raised in today's cyber world. In response, researchers have developed security mechanisms to protect users and system resources from misuse. However, attackers are increasingly finding new ways to avoid detection by these mechanisms. As a consequence, new and improved security mechanisms are in constant need of development, which may be a challenge in terms of cost. Increased security may also impair the usability of the web-based services. As such, there is often a clear trade-off between the variables of: security level, usability and cost. This paper therefore proposes approaches that can effectively handle this trade-off.