Date post: | 16-Dec-2015 |
Category: |
Documents |
Upload: | eliana-coxon |
View: | 215 times |
Download: | 1 times |
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
2
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]
3
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]
3
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!
44
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
5
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
6
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
7
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
8
• 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
10
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
11
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
12
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
13
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
14
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%
15
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
16
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