Attacks against Wireless Sensor Networks (WSNs) are becoming more and more frequent. Due to the sensitive nature of these networks, appropriate security mechanisms should therefore be developed. This seminars introduces a fault-tolerant approach for WSNs. The basic idea to detect malicious nodes is to proceed to intermediate verification of the signed messages within the communication paths. A tolerance state is defined in which a set of tests are performed on the malicious node in order to definitely reject it or rehabilitate it.