1. A system for detecting file upload vulnerabilities in web applications, the system comprising: a black-box tester configured to upload, via a file upload interface exposed by a web application, a file together with a signature associated with the file; and an execution monitor configured to receive information provided by instrumentation instructions within the web application during the execution of the web application, wherein the execution monitor is configured to recognize the signature of the uploaded file as indicating that the uploaded file was uploaded by the black-box tester, and wherein the execution monitor is configured to use any of the information to make at least one predefined determination assessing the vulnerability of the web application to a file upload exploit.