—It is crucial to detect zero-day polymorphic worms and to generate signatures at network gateways or honeynets so that we can prevent worms from propagating at their early phase. However, most existing network-based signatures are specific to exploit and can be easily evaded. In this paper, we propose generating vulnerability-driven signatures at network level without any host-level analysis of worm execution or vulnerable programs. As the first step, we design a network-based length-based signature generator (LESG) for the worms exploiting buffer overflow