This paper presents a technique for vulnerability detection in C programs. It is based on a vulnerability formal model called "Vulnerability Detection Conditions" (VDCs). This model is used together with passive testing techniques for the automatic detection of vulnerabilities. The proposed technique has been implemented in a dynamic code analysis tool, TestInv-Code, which detects the presence of vulnerabilities on a given code, by checking dynamically the VDCs on the execution traces of the given program. The tool has been applied to several C applications containing some well known vulnerabilities to illustrate its effectiveness. It has also been compared with existing tools in the market, showing promising performances