Vulnerability detection tools are frequently considered the silver-bullet for detecting vulnerabilities in web services. However, research shows that the effectiveness of most of those tools is very low and that using the wrong tool may lead to the deployment of services with undetected vulnerabilities. In this paper we propose a benchmarking approach to assess and compare the effectiveness of vulnerability detection tools in web services environments. This approach was used to define a concrete benchmark for SQL Injection vulnerability detection tools. This benchmark is demonstrated by a real example of benchmarking several widely used tools, including four penetration-testers, three static code analyzers, and one anomaly detector. Results show that the benchmark accurately portrays the effectiveness of vulnerability detection tools and suggest that the proposed approach can be applied in the field.