Abstract. Many worm detectors have been proposed and are being deployed, but the literature does not clearly indicate which one is the best. New worms such as IKEE.B (also known as the iPhone worm) continue to present new challenges to worm detection, further raising the question of how effective our worm defenses are. In this paper, we identify six behavior-based worm detection algorithms as being potentially capable of detecting worms such as IKEE.B, and then measure their performance across a variety of environments and worm scanning behaviors, using common parameters and metrics. We show that the underlying network trace used to evaluate worm detectors significantly impacts their measured performance. An environment containing substantial gaming and file sharing traffic can cause the detectors to perform poorly. No single detector stands out as suitable for all situations. For instance, connection failure monitoring is the most effective algorithm in many environments, but it f...