Web sites are often a mixture of static sites and programs that integrate relational databases as a back-end. Software that implements Web sites continuously evolve to meet ever-changing user needs. As a Web sites evolve, new versions of programs, interactions and functionalities are added and existing ones are removed or modified. Web sites require configuration and programming attention to assure security, confidentiality, and trustiness of the published information. During evolution of Web software, from one version to the next one, security flaws may be introduced, corrected, or ignored. This paper presents an investigation of the evolution of security vulnerabilities as detected by propagating and combining granted authorization levels along an inter-procedural control flow graph (CFG) together with required security levels for DB accesses with respect to SQL-injection attacks. The paper reports results about experiments performed on 31 versions of phpBB, that is a publicly available bulletin board written in PHP, version 1.0.0 (9547 LOC) to version 2.0.22 (40663 LOC) have been considered as a case study. Results show that the vulnerability analysis can be used to observe and monitor the evolution of security vulnerabilities in subsequent versions of the same software package. Suggestions for further research are also presented.