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Do You See What I See? Differential Treatment of Anonymous Users

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Abstract: The utility of anonymous communication is undermined by a growing number of websites treating users of such services in a degraded fashion. The second-class treatment of anonymous users ranges from outright rejection to limiting their access to a subset of the service’s functionality or imposing hurdles such as CAPTCHA -solving. To date, the observation of such practices has relied upon anecdotal reports catalogued by frustrated anonymity users. We present a study to methodically enumerate and characterize, in the context of Tor, the treatment of anonymous users as second-class Web citizens.

We focus on first-line blocking: at the transport layer, through reset or dropped connections; and at the application layer, through explicit blocks served from website home pages. Our study draws upon several data sources: comparisons of Internet-wide port scans from Tor exit nodes versus from control hosts; scans of the home pages of top-1,000 Alexa websites through every Tor exit; and analysis of nearly a year of historic HTTP crawls from Tor network and control hosts. We develop a methodology to distinguish censorship events from incidental failures such as those caused by packet loss or network outages, and incorporate consideration of the endemic churn in web-accessible services over both time and geographic diversity. We find clear evidence of Tor blocking on the Web, including 3.5% of the top-1,000 Alexa sites. Some blocks specifically target Tor, while others result from fate-sharing when abuse-based automated blockers trigger due to misbehaving Web sessions sharing the same exit node.

Bio: Sheharbano Khattak is a PhD student and Research Assistant in the Security and NetOS groups of the Computer Lab, University of Cambridge, under the supervision of Dr. Steven J. Murdoch, Prof. Jon Crowcroft and Prof. Ross Anderson. She is externally advised by Prof. Vern Paxson at UC Berkeley. Sheharbano is a member of Robinson College and an Honorary Cambridge Trust Scholar. She likes to work on network measurement and security in isolation, and various combinations of these. Currently she studies the effects of online censorship from a number of different aspects: how it’s done, how it can be stopped, what its effects are, and the evolving shape of the ecosystem of government/policy-based censorship in particular. Previously she worked on Intrusion Detection Systems and Internet malware with a focus on botnets.

This talk is part of the Computer Laboratory Security Seminar series.

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