One Bad Apple Spoils the Bunch: Exploiting P2P Applications to Trace and Profile Tor Users

TL;DR

This paper demonstrates how linkability in Tor can be exploited to trace and profile users, revealing thousands of IPs and exposing behaviors, especially related to BitTorrent usage, over a 23-day period.

Contribution

It introduces practical attacks exploiting linkability in Tor to trace insecure applications like BitTorrent, revealing user IPs and behaviors in real-world scenarios.

Findings

Linkability allows tracing 193% more streams, including 27% from 'secure' browsers.

Successfully traced 9% of Tor streams via exit nodes.

Identified over 10,000 user IPs and analyzed their content and geographic distribution.

Abstract

Tor is a popular low-latency anonymity network. However, Tor does not protect against the exploitation of an insecure application to reveal the IP address of, or trace, a TCP stream. In addition, because of the linkability of Tor streams sent together over a single circuit, tracing one stream sent over a circuit traces them all. Surprisingly, it is unknown whether this linkability allows in practice to trace a significant number of streams originating from secure (i.e., proxied) applications. In this paper, we show that linkability allows us to trace 193% of additional streams, including 27% of HTTP streams possibly originating from "secure" browsers. In particular, we traced 9% of Tor streams carried by our instrumented exit nodes. Using BitTorrent as the insecure application, we design two attacks tracing BitTorrent users on Tor. We run these attacks in the wild for 23 days and reveal 10,000 IP addresses of Tor users. Using these IP addresses, we then profile not only the BitTorrent downloads but also the websites visited per country of origin of Tor users. We show that BitTorrent users on Tor are over-represented in some countries as compared to BitTorrent users outside of Tor. By analyzing the type of content downloaded, we then explain the observed behaviors by the higher concentration of pornographic content downloaded at the scale of a country. Finally, we present results suggesting the existence of an underground BitTorrent ecosystem on Tor.