Who Carries Tor? Measuring Bandwidth-Weighted Transit Concentration
Authors: Isabelle Wang (Smith College), Shinyoung Cho (Smith College)
Year: 2026
Issue: 2
Pages: 37–41
Abstract: The Tor network remains vulnerable to the underlying inter-domain routing infrastructure. A small set of Autonomous Systems (ASes) act as critical intermediaries and may gain visibility into substantial fractions of inbound and outbound Tor traffic, creating potential chokepoints that could facilitate traffic correlation attacks. To quantify this risk, we adapt the AS Hegemony metric and extend it by introducing a Symbolic ASN to represent the Tor network as a logical destination and incorporating consensus bandwidth weighting to reflect actual traffic patterns. Using this methodology, we conduct a longitudinal study spanning 2015–2025 and a country-level analysis of Tor’s AS-level transit dependency. Our findings reveal persistent concentration in a small number of transit ASes, a dramatic surge of SURF (AS1103) to over 50% exit hegemony in 2024, and significant sensitivity to the distribution of relay capacity across ASes at the country level. We further propose a diversity-aware relay placement heuristic to guide future mitigation efforts.
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