Systems based on the use of watermarking techniques are seen in recent times as the most interesting solutions to provide means of protecting Intellectual Property Rights in electronic commerce with digital contents environments. A key aspect of this secure e-commerce environment is security. It is important to be able to evaluate and to know the security level and the risks of die system providing these type of services, specially in open networks like Internet. 'Attack trees' technique has proved valid as a methodology for studying and analyzing the security of these distributed systems, and as a mean to compare the security level of similar schemes.