The security of Supervisory Control and Data Acquistition systems (SCADA) has become these last years, a major worldwide concern. Indeed, several incidents and cyber-attacks stressed the emergency to make more efforts to secure these systems which manage important economical infrastructures. The increasing use of wireless sensors also brings their security vulnerabilities. Therefore, several communication protocols were developed to meet real time and security requirements needed by this kind of systems. WirelessHART is the first approved international standard for industrial wireless devices. It implements several mechanisms to ensure hop-by-hop and end-to-end security. However, despite these mechanisms, it remains possible for an attacker to conduct an attack against such wireless networks. In this paper, we give the first description of a Sybil attack specially tailored to target WirelessHART based SCADA systems. This attack can lead to harmful consequences such as disturbing the infrastructure functioning, interrupting it or more again causing its destruction (overheating of a nuclear reactor).