In recent years Web-based systems have become extremely popular and, nowadays, they are used in critical environments such as financial, medical, and military systems. As the use of Web applications for security-critical services has increased, the number and sophistication of attacks against these applications have grown as well. For this reason it is essential to be able to prove that the target Web-based system implements its designed security requirements avoiding known vulnerabilities in HTTP-based solutions. To reach this aim, we can rely on several testing techniques and mainly on security passive testing approach that is becoming increasingly important to security-relevant aspects into web based software systems. This article describes the application of the TestInv-P passive testing tool as part of the testing phase of TXT etourism Web application. TestInv-P is a passive testing tool that monitors communication traces of an application during run-time and verifies whether it satisfies certain securityrelated invariants derived from SHIELDS models. 1