Memory-based DoS and Deanonymization Attacks on Tor DCAPS Seminar October 11 th, 2013 Rob Jansen U.S. Naval Research Laboratory *Joint with Aaron Johnson, Florian Tschorsch, Björn Scheuermann

The Tor Anonymity Network torproject.org

How Tor Works

Tor protocol aware

Tor Flow Control exit entry

Tor Flow Control One TCP Connection Between Each Relay, Multiple Circuits exit entry

Tor Flow Control One TCP Connection Between Each Relay, Multiple Circuits Multiple Application Streams exit entry

Tor Flow Control No end-to-end TCP! exit entry

Tor Flow Control Tor protocol aware exit entry

Tor Flow Control Packaging End Delivery End exit entry

Tor Flow Control Packaging End Delivery End exit entry

Tor Flow Control 1000 Cell Limit SENDME Signal Every 100 Cells exit entry

Outline ● The Sniper Attack – Low-cost memory consumption attack that disables arbitrary Tor relays ● Deanonymizing Hidden Services – Using DoS attacks for deanonymization ● Countermeasures

The Sniper Attack Start Download Request exit entry

The Sniper Attack Reply DATA exit entry

The Sniper Attack Package and Relay DATA DATA exit entry

The Sniper Attack DATA Stop Reading from Connection DATA R exitentry

The Sniper Attack DATA R exit entry Flow Window Closed

The Sniper Attack DATA Periodically Send SENDME SENDME R DATA exit entry

The Sniper Attack DATA Periodically Send SENDME SENDME R DATA exit entry Flow Window Opened

The Sniper Attack DATA R exit entry DATA Out of Memory, Killed by OS

The Sniper Attack DATA R exit entry DATA Use Tor to Hide

Memory Consumed over Time

Mean RAM Consumed, 50 Relays

Mean BW Consumed, 50 Relays

Speed of Sniper Attack DirectAnonymous Relay GroupsSelect %1 GiB8 GiB1 GiB8 GiB Top Guard1.7 Top 5 Guards6.5 Top 20 Guards19 Top Exit3.2 Top 5 Exits13 Top 20 Exits35 Path Selection Probability ≈ Network Capacity

Speed of Sniper Attack DirectAnonymous Relay GroupsSelect %1 GiB8 GiB1 GiB8 GiB Top Guard1.70:010:180:020:14 Top 5 Guards6.50:081:030:121:37 Top 20 Guards190:455:581:078:56 Top Exit3.20:010:080:010:12 Top 5 Exits130:050:370:070:57 Top 20 Exits350:293:500:445:52 Time (hours:minutes) to Consume RAM

Speed of Sniper Attack DirectAnonymous Relay GroupsSelect %1 GiB8 GiB1 GiB8 GiB Top Guard1.70:010:180:020:14 Top 5 Guards6.50:081:030:121:37 Top 20 Guards190:455:581:078:56 Top Exit3.20:010:080:010:12 Top 5 Exits130:050:370:070:57 Top 20 Exits350:293:500:445:52 Time (hours:minutes) to Consume RAM

Speed of Sniper Attack DirectAnonymous Relay GroupsSelect %1 GiB8 GiB1 GiB8 GiB Top Guard1.70:010:180:020:14 Top 5 Guards6.50:081:030:121:37 Top 20 Guards190:455:581:078:56 Top Exit3.20:010:080:010:12 Top 5 Exits130:050:370:070:57 Top 20 Exits350:293:500:445:52 Time (hours:minutes) to Consume RAM

Outline ● The Sniper Attack – Low-cost memory consumption attack that disables arbitrary Tor relays ● Deanonymizing Hidden Services – Using DoS attacks for deanonymization ● Countermeasures

Hidden Services HS User wants to hide service

Hidden Services entry IP HS chooses and publishes introduction point IP HS

Hidden Services entry IP HS Learns about HS on web

entry Hidden Services entry IP HS Builds Circuit to Chosen Rendezvous Point RP RP

entry Hidden Services entry IP HS Notifies HS of RP through IP RP entry RP

entry Hidden Services entry IP HS RP

entry Hidden Services entry IP HS Build New Circuit to RP RP entry RP

entry Hidden Services entry IP HS Communicate! RP entry RP

entry Deanonymizing Hidden Services HS RP

entry Deanonymizing Hidden Services HS RP Also runs a guard relay

entry Deanonymizing Hidden Services entry HS RP Build New Circuit to RP

entry Deanonymizing Hidden Services entry HS RP S&P 2006, S&P 2013

entry Deanonymizing Hidden Services entry HS RP S&P 2013 PADDIN G Send 50 Padding Cells

entry Deanonymizing Hidden Services entry HS RP Identify HS entry if cell count = 52 S&P 2013

entry Deanonymizing Hidden Services entry HS RP Sniper Attack, or any other DoS

entry Deanonymizing Hidden Services HS RP Choose new Entry Guard

entry Deanonymizing Hidden Services HS RP

entry Deanonymizing Hidden Services HS RP S&P 2006, S&P 2013

entry Deanonymizing Hidden Services HS RP Send 50 Padding Cells S&P 2013 PADDIN G

entry Deanonymizing Hidden Services HS RP Identify HS if cell count = 53 S&P 2013

Outline ● The Sniper Attack – Low-cost memory consumption attack that disables arbitrary Tor relays ● Deanonymizing Hidden Services – Using DoS attacks for deanonymization ● Countermeasures

Countermeasures ● Sniper Attack Defenses – Authenticated SENDMEs – Queue Length Limit – Adaptive Circuit Killer ● Deanonymization Defenses – Entry-guard Rate-limiting – Middle Guards

Questions? cs.umn.edu/~jansen think like an adversary

Speed of Deanonymization Guard BW (MiB/s) Guard Probability (%) Average # Rounds Average # Sniped Average Time (h) 1 GiB Average Time (h) 8 GiB GiB/s Relay Can Deanonymize HS in about a day

Circuit Killer Defense

The Sniper Attack exit entry exitentry Single Adversary

The Sniper Attack exit entry exitentry Anonymous Tunnel

The Sniper Attack exit entry exitentry

The Sniper Attack exit entry exitentry DATA

The Sniper Attack exit entry exitentry DATA R

The Sniper Attack exit entry exitentry DATA R Flow Window Closed

The Sniper Attack exit entry exitentry DATA R R

The Sniper Attack exit entry exitentry DATA R R

The Sniper Attack exit entry exitentry DATA R R Killed by OS DATA