Compromising Anonymity Using Packet Spinning

We present a novel attack targeting anonymizing systems. The attack involves placing a malicious relay node inside an anonymizing system and keeping legitimate nodes “busy.” We achieve this by creating circular circuits and injecting fraudulent packets, crafted in a way that will make them spin an arbitrary number of times inside our artificial loops. At the same time we inject a small number of malicious nodes that we control into the anonymizing system. By keeping a significant part of the anonymizing system busy spinning useless packets, we increase the probability of having our nodes selected in the creation of legitimate circuits, since we have more free capacity to route requests than the legitimate nodes. This technique may lead to the compromise of the anonymity of people using the system. To evaluate our novel attack, we used a real-world anonymizing system, TOR. We show that an anonymizing system that is composed of a series of relay nodes which perform cryptographic operations is vulnerable to our packet spinning attack. Our evaluation focuses on determining the cost we can introduce to the legitimate nodes by injecting the fraudulent packets, and the time required for a malicious client to create n-length TOR circuits. Furthermore we prove that routers that are involved in packet spinning do not have the capacity to process requests for the creation of new circuits and thus users are forced to select our malicious nodes for routing their data streams.