Predicting Tor Path Compromise by Exit Port IEEE WIDA 2009December 16, 2009 Kevin Bauer, Dirk Grunwald, and Douglas Sicker University of Colorado Client Destination Host Entry Guard Middle Router Exit Router Directory Server Circuit Tor Network

Tor: Anonymity for TCP Applications 1 Client Destination Host Entry Guard Middle Router Exit Router Directory Server Circuit Router List Tor provides anonymity for TCP by tunneling traffic through a virtual circuit of three Tor routers using layered encryption 1 First hop knows the client Last hop knows the destination Tor Network Colluding entry and exit routers can use simple timing analysis to de-anonymize the client and destination [Serjantov et al., 2003; Levine et al., 2004]

Prior Attacks Against Tor 2 Client Destination Host Entry Guard Middle Router Exit Router Directory Server Circuit Router List Prior work showed that the likelihood of circuit compromise in Tor is relatively high [Bauer et al., 2007] 2 First hop knows the client Last hop knows the destination Tor Network 1. Clients choose Tor routers in proportion to their bandwidths 2. Tor routers self-advertise their bandwidth capacities High BW routers chosen most often Routers can lie!

33 We extend prior work by investigating whether certain applications are more vulnerable to attack than others We hypothesize that traffic destined for ports with little bandwidth is more vulnerable to circuit compromise Our Contribution We observe that the bandwidth available for different applications is not uniformly distributed among exit Tor routers

Talk Outline Background on path selection in Tor Experimental setup Experimental results – Exit bandwidth is not uniformly distributed – Long-lived traffic requires “stable” routers Toward solutions Future work Summary and conclusions 4

Path Selection in Tor Clients choose Tor routers in proportion to their bandwidth capacities To reduce the risk of path compromise, Tor clients choose their circuits very carefully Circuit construction rules A router may only be used once per circuit Only one router per /16 network and two routers per IP address First router must be an entry guard The exit router must allow connections to the traffic’s destination host and port 5 Mitigates risk of choosing adversary controlled routers Mitigates the “predecessor attack” Ensures traffic can be delivered

Path Selection: Exit Policies Tor allows exit routers to specify their own exit policies Can be used to help router operators manage risk of abuse [Bauer et al., 2008] Possible Tor router configurations – Non-exit: Router is not allowed to connect to any (non-Tor) Internet host – Exit: May connect to designated port numbers (and hosts) on the Internet 6 Client Destination Host Entry Guard Exit Router Middle Router

Applications with persistent sessions (SSH, FTP) require special routers that have been alive for a long time Marked as Stable by the directory servers – Stable router is in the top half of all routers in terms of mean time between failures – Or alive for at least 30 days Path Selection: Stable Paths 7

Experimental Evaluation: Setup We simulate Tor’s router selection algorithm to study how certain applications may be more vulnerable to circuit compromise Fuel simulations with real Tor router data from the directory servers (May 31, 2009 snapshot) – 1,444 total routers with MB total bandwidth – 770 “stable” routers with MB total bandwidth Simulation details – Generate 10,000 circuits for applications (default port): FTP (21), SSH (22), Telnet (23), SMTP (25), HTTP (80), POP3 (110), HTTPS (443), Kazaa P2P (1214), BitTorrent tracker (6969), Gnutella P2P (6346), and eDonkey P2P (4661) – Add malicious routers (10 MB/s BW) and count compromised circuits 8

Experimental Evaluation: Results 9 SMTP (outgoing ) and peer-to-peer file sharing applications are more vulnerable to circuit compromise 6 routers (with 60 MB) make up 12% of the total bandwidth The number of circuits compromised increases as more malicious routers are injected into the network Fraction of circuits that are compromised for each application’s default exit port

Exit Bandwidth Distribution is Skewed 10 SMTP and peer-to-peer applications have fewest routers and least amount of exit bandwidth Distribution of exit bandwidth by default exit port number Fraction of circuits that are compromised for each application’s default exit port

Long-Lived Traffic Needs “Stable” Routers Applications with persistent sessions require “stable” routers Only 770/1,444 routers are Stable Slightly higher compromise rate than HTTP/HTTPS/Telnet/POP3 11 Distribution of exit bandwidth by default exit port number Fraction of circuits that are compromised for each application’s default exit port

Only the Exit Router is Malicious If only the exit router is malicious, an attacker could still learn significant identifying information – i.e., Login credentials HTTP – 6 malicious routers: Controls exit router 33.6% of the time – 16 malicious routers: Controls exit router 56.5% of the time FTP – 6 malicious routers: Controls exit router 46.7% of the time – 16 malicious routers: Controls exit router 70.7% of the time This is a very real threat, since many popular websites still do not provide TLS-protected logins 12

Toward Solutions One solution is to give users the ability to manage their risk of attack Prior work proposed that users tune the router selection between bandwidth-weighted and uniform router selection [Snader and Borisov, 2008] – Allows users to trade-off between strong anonymity and strong performance However, it remains necessary to balance the traffic load over the available bandwidth General solutions to this attack is an open problem 13 Uniform router selection: c > 1 malicious routers E > 0 is number exit routers N > 1 number total routers Only 0.09% of BitTorrent tracker circuits compromised Compare to 18.5%

Future Work: Selective DoS Attacks Extend this work to consider selective denial-of-service attacks – Attack strategy: If an adversary does not control the endpoints of a given circuit, they disrupt the circuit, causing it to be rebuilt 14 Fraction of circuits that are compromised for each application’s default exit port Initial results with selective denial-of-service Effects of bandwidth disparities are magnified SMTP and peer-to-peer applications show extremely high compromise rate (68-93%) with only 6 malicious routers

Summary and Conclusions We demonstrated our hypothesis that certain applications are more vulnerable than others to circuit compromise in Tor Through a simulation study driven by data obtained from the real Tor network, we found that SMTP and peer-to-peer file sharing applications are most vulnerable We suggest that concerned users tune the router selection bias to control the risk of path compromise 15 Client Destination Host Entry Guard Middle Router Exit Router Directory Server Circuit Tor Network