This paper describes a PU learning (Positive and Unlabeled learning) based system for potential URL attack detection. Previous machine learning based solutions for this task mainly formalize it as a supervised learning problem. However, in some scenarios, the data obtained always contains only a handful of known attack URLs, along with a large number of unlabeled instances, making the supervised learning paradigms infeasible. In this work, we formalize this setting as a PU learning problem, and solve it by combining two different strategies (two-stage strategy and cost-sensitive strategy). Experimental results show that the developed system can effectively find potential URL attacks. This system can either be deployed as an assistance for existing system or be employed to help cyber-security engineers to effectively discover potential attack mode so that they can improve the existing system with significantly less efforts.