1. A system for detecting security vulnerabilities in web applications, the system comprising: a black-box tester configured to provide a payload to a web application during a first interaction with the web application at a computer server, where the payload includes a payload instruction and an identifier; and a hardware processor including an execution engine configured to detect the identifier within the payload received during an interaction with the web application subsequent to the first interaction, and determine, responsive to detecting the identifier within the payload, whether the payload instruction underwent a security check prior to execution of the payload instruction, wherein the black-box tester is configured to report that the web application is vulnerable to a stored cross-site scripting attack where the execution engine detects the identifier within the payload and determines that the payload instruction did not undergo a security check prior to the execution of the payload instruction.