In bug tracking system, the high volume of incoming bug reports poses a serious challenge to project managers. Triaging these bug reports manually consumes time and resources which leads to delaying the resolution of important bugs. StackOverflow is the most popular crowdsourcing Q&A community with plenty of bug-related posts. In this paper, we propose a new method to evaluate bug severity by exploiting available quantitative crowd-based knowledge from the bug-related posts in StackOverflow. Two typical type of projects' bug repositories are studied here, e.g. Mozilla and Eclipse. Our study shows that using crowd-based knowledge is able capture the technical merits as well as the programming solutions and it has the potential to optimize the existing evaluation methods of bug severity by incorporating the crowd-based knowledge from a third-party in future.