Characterizing the Nature and Dynamics of Tor Exit Blocking

Website Maintenance Alert

Due to scheduled maintenance, the USENIX website will not be available on Tuesday, December 17, from 10:00 am to 2:00 pm Pacific Daylight Time (UTC -7). We apologize for the inconvenience.

If you are trying to register for Enigma 2020, please complete your registration before or after this time period.


Rachee Singh, University of Massachusetts – Amherst; Rishab Nithyanand, Stony Brook University; Sadia Afroz, University of California, Berkeley and International Computer Science Institute; Paul Pearce, UC Berkeley; Michael Carl Tschantz, International Computer Science Institute; Phillipa Gill, University of Massachusetts – Amherst; Vern Paxson, University of California, Berkeley and International Computer Science Institute


Facing abusive traffic from the Tor anonymity network, online service providers discriminate against Tor users. In this study, we characterize not only the extent of such discrimination but also the nature of the undesired traffic originating from the Tor network—a task complicated by Tor’s need to maintain user anonymity. We address this challenge by leveraging multiple independent data sources: email complaints sent to exit operators, commercial IP blacklists, webpage crawls via Tor, and privacy-sensitive measurements of our own Tor exit nodes. As part of our study, we also develop methods for classifying email complaints and an interactive crawler to find subtle forms of discrimination, and deploy our own exits in various configurations to understand which are prone to discrimination. We find that conservative exit policies are ineffective in preventing the blacklisting of exit relays. However, a majority of the attacks originating from Tor generate high traffic volume, suggesting the possibility of detection and prevention without violating Tor users’ privacy.

Open Access Media

USENIX is committed to Open Access to the research presented at our events. Papers and proceedings are freely available to everyone once the event begins. Any video, audio, and/or slides that are posted after the event are also free and open to everyone. Support USENIX and our commitment to Open Access.

@inproceedings {203856,
author = {Rachee Singh and Rishab Nithyanand and Sadia Afroz and Paul Pearce and Michael Carl Tschantz and Phillipa Gill and Vern Paxson},
title = {Characterizing the Nature and Dynamics of Tor Exit Blocking},
booktitle = {26th {USENIX} Security Symposium ({USENIX} Security 17)},
year = {2017},
isbn = {978-1-931971-40-9},
address = {Vancouver, BC},
pages = {325--341},
url = {},
publisher = {{USENIX} Association},
month = aug,

Presentation Video