Abstract-Fast spreading worms are a reality, as amply demonstrated by worms such as Slammer, which reached its peak propagation in a matter of minutes. With these kinds of fast spreading worms, the traditional approach of signature-based detection is no longer sufficient. Specifically, these worms can infect all vulnerable hosts well before a signature is available. To counter them, we must devise fast detection algorithms that can detect new worms without signatures as they first begin to appear. We present the design and evaluation of such an algorithm in this paper.The key to the algorithm is the identification of certain invariant characteristics of worm propagation. Specifically, we are able to demonstrate using real network traces how worm propagation can perturb the arrival process distribution of unsolicited packets. Our algorithm employs a novel two-step procedure that combines a first stage change point detection with a second stage growth rate inference to confirm the existence of a worm.To evaluate the algorithm, we have applied it to multi-year network traces that cover many of the major worm outbreaks in recent years, including Slammer, Witty, Nimda and Blaster. In all cases, the new algorithm is able to detect the worm within a very short time, well before significant infection has taken place.