Scan Me If You Can: Understanding and Detecting Unwanted Vulnerability Scanning

Web vulnerability scanners (WVS) are an indispensable tool for penetration testers and developers of web applications, allowing them to identify and fix low-hanging vulnerabilities before they are discovered by attackers. Unfortunately, malicious actors leverage the very same tools to identify and exploit vulnerabilities in third-party websites. Existing research in the WVS space is largely concerned with how many vulnerabilities these tools can discover, as opposed to trying to identify the tools themselves when they are used illicitly. In this work, we design a testbed to characterize web vulnerability scanners using browser-based and network-based fingerprinting techniques. We conduct a measurement study over 12 web vulnerability scanners as well as 159 users who were recruited to interact with the same web applications that were targeted by the evaluated WVSs. By contrasting the traffic and behavior of these two groups, we discover tool-specific and type-specific behaviors in WVSs that are absent from regular users. Based on these observations, we design and build ScannerScope, a machine-learning-based, web vulnerability scanner detection system. ScannerScope consists of a transparent reverse proxy that injects fingerprinting modules on the fly without the assistance (or knowledge) of the protected web applications. Our evaluation results show that ScannerScope can effectively detect WVSs and protect web applications against unwanted vulnerability scanning, with a detection accuracy of over 99% combined with near-zero false positives on human-visitor traffic. Finally, we show that the asynchronous design of Scanner-Scope results in a negligible impact on server performance and demonstrate that its classifier can resist adversarial ML attacks launched by sophisticated adversaries.