Wireless sensor networks, applied for monitoring physical environments, have recently emerged as an important application of the ad hoc networks paradigm. Low processing power and wireless connectivity make such networks vulnerable to various types of attacks. The dense deployment of sensors along with the protocol deficiencies, make broadcast attacks feasible. The current solutions for these types of attacks are mainly cryptographic and suffer from either heavy computational complexity or lack of mobility support. In this paper we propose an energy efficient method to fortify sensor networks against broadcast attacks using some key features of such networks like small transmission range and low reception-sensitivity. The main idea relies on modifying the reply timings of current MAC sub-layer protocols and embedding a two or three-way handshake protocol in broadcast scenarios so that the adversary receives a negative feedback from the network in response to his greediness in infecting nodes. The simulations confirm the efficiency of this countermeasure applied to the IEEE 802.15.4.