We present a novel, practical, and effective mechanism for identifying the IP address of Tor clients. We approximate an almost-global passive adversary (GPA) capable of eavesdropping anywhere in the network by using LinkWidth, a novel bandwidth-estimation technique. LinkWidth allows network edge-attached entities to estimate the available bandwidth in an arbitrary Internet link without a cooperating peer host, router, or ISP. By modulating the bandwidth of an anonymous connection (e.g., when the destination server or its router is under our control), we can observe these fluctuations as they propagate through the Tor network and the Internet to the end-user's IP address. Our technique exploits one of the design criteria for Tor by allowing well-provisioned adversaries to effectively become GPAs.

Although timing-based attacks have been demonstrated against non- timing-preserving anonymity networks, they have depended either on a global passive adversary or on the compromise of a substantial number of Tor nodes. Our technique does not require compromise of any Tor nodes. We demonstrate the effectiveness of our approach in tracking the IP address of Tor users in a series of experiments. Even for an under-provisioned adversary with only two network vantage points, we can accurately identify the end user (IP address) in many cases. Furthermore, we show that a well-provisioned adversary, using a topological map of the network, can trace-back the path of an anonymous user in under 20 minutes. Finally, we can trace an anonymous Location Hidden Service in approximately 120 minutes.