Do You See What I See? Differential Treatment of Anonymous Users presented at NDSS 2016

by David Fifield, Damon Mccoy, Steven J. Murdoch, Vern Paxson, Sadia Afroz, Sheharbano Khattak, Mobin Javed, Srikanth Sundaresan,

Summary : 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 conduct the first study to methodically enumerate and characterize the treatment of anonymous users as second-class Web citizens in the context of Tor.
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; longitudinal scans of 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 churn in web-accessible services over both time and geographic diversity. We find clear evidence of Tor blocking on the Web, including 3% of the 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.