In the recent years Mobile Ad-hoc Network is emerging as a very popular technology in the wireless network. Because of its dynamic topology and its infrastructure less architecture, it is vulnerable to various types of attack. Providing security against the intruder is a challenging task in MANET. In this paper, we present the Modified Enhanced Adaptive Acknowledgment (EAACK) based Intrusion Detection system that is used to mitigate the attacks. The proposed scheme is also used to overcome the limitations of watchdog. Elliptic Curve DSA (ECDSA) is used to authenticate the acknowledgment packets used in the proposed technique in order to improve the security level. When compared to the existing method, the proposed method achieves smaller end to end delay and higher throughput performance.