Code reusing is a common practice in software development due to its various benefits. Such a practice, however, may also cause large scale security issues since one vulnerability may appear in many different software due to cloned code fragments. The well known concept of relying on software diversity for security may also be compromised since seemingly different software may in fact share vulnerable code fragments. Although there exist efforts on detecting cloned code fragments, there lack solutions for formally characterizing their specific impact on security. In this paper, we revisit the concept of software diversity from a security viewpoint. Specifically, we define the novel concept of common attack surface to model the relative degree to which a pair of software may be sharing potentially vulnerable code fragments. To implement the concept, we develop an automated tool, CASFinder, in order to efficiently identify common attack surface between any given pair of software with minimum human intervention. Finally, we conduct experiments by applying our tool to real world open source software applications. Our results demonstrate many seemingly unrelated software applications indeed share significant common attack surface.