Predicting Tor Path Compromise by Exit Port IEEE WIDA 2009December 16, 2009 Kevin Bauer, Dirk Grunwald, and Douglas Sicker University of Colorado Client.

Predicting Tor Path Compromise by Exit Port

IEEE WIDA 2009 December 16, 2009

Kevin Bauer, Dirk Grunwald, and Douglas SickerUniversity of Colorado

Client

Destination Host

Entry Guard

Middle Router

Exit Router

Directory ServerCircuit

Tor Network

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Tor: Anonymity for TCP Applications

Client

Destination Host

Entry Guard

Middle Router

Exit Router

Directory ServerCircuit

Router List

Tor provides anonymity for TCP by tunneling traffic through a virtual circuit of three Tor routers using layered encryption

2

First hop knows the client

Last hop knowsthe 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]

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Prior Attacks Against Tor

Client

Destination Host

Entry Guard

Middle Router

Exit Router

Directory ServerCircuit

Router List

Prior work showed that the likelihood of circuit compromise in Tor is relatively high [Bauer et al., 2007]

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First hop knows the client

Last hop knowsthe destination

Tor Network

1. Clients choose Tor routers in proportion to their bandwidths

2. Tor routers self-advertise their bandwidth capacities

High BW routerschosen most often

Routers can lie!

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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

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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

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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

Mitigates risk of choosingadversarycontrolledrouters

Mitigates the“predecessorattack”Ensures trafficcan be delivered

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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

Client

Destination Host

Entry Guard

Exit RouterMiddle Router

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• 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

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 403.3 MB total bandwidth– 770 “stable” routers with 326.9 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 6 - 106 malicious routers (10 MB/s BW) and count compromised circuits 9

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Experimental Evaluation: Results

SMTP (outgoing E-mail) 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 moremalicious routers are injected into the network

Fraction of circuits that are compromised for each application’s default exit port

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Exit Bandwidth Distribution is Skewed

SMTP and peer-to-peer applications have fewestrouters 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

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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

Distribution of exit bandwidth by default exit port number

Fraction of circuits that are compromised for each application’s default exit port

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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

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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

Uniform router selection:c > 1 malicious routersE > 0 is number exit routersN > 1 number total routers

Only 0.09% of BitTorrent tracker circuits compromised

Compare to 18.5%

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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 rebuiltFraction 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 highcompromise rate (68-93%) with only 6 malicious routers

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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

Client

Destination Host

Entry Guard

Middle Router

Exit Router

Directory ServerCircuit

Tor Network