We present WebSOS, a novel overlay-based architecture that provides guaranteed access to a web server that is targeted by a denial of service (DoS) attack. Our approach exploits two key characteristics of the web environment: its design around a human-centric interface, and the extensibility inherent in many browsers through downloadable "applets." We guarantee access to a web server for a large number of previously unknown users, without requiring pre-existing trust relationships between users and the system.Our prototype requires no modifications to either servers or browsers, and makes use of graphical Turing tests, web proxies, and client authentication using the SSL/TLS protocol, all readily supported by modern browsers. We use the WebSOS prototype to conduct a performance evaluation over the Internet using PlanetLab, a testbed for experimentation with network overlays. We determine the end-to-end latency using both a Chord-based approach and our shortcut extension. Our evaluation shows the latency increase by a factor of 7 and 2 respectively, confirming our simulation results.
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