It is crucial to detect zero-day polymorphic worms and to generate signatures at the edge network gateways or honeynets so that we can prevent the worms from propagating at their early phase. However, most existing network-based signatures generated are not vulnerability-based and can be easily evaded by attacks. In this paper, we propose generating vulnerability-based signatures on the 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 worms based on buffer overflow vulnerabilities. The signatures generated are intrinsic to buffer overflows, and are very hard for attackers to evade. We further prove the attack resilience bounds even under worst case attacks with deliberate noise injection. Moreover, LESG is fast and noise-tolerant and has efficient signature matching. Evaluation based on real-world vulnerabilities of various protocols and real network traffic demonstrates that LESG is promising in achieving these goals.