  User-generated content sites routinely block contributions from users of
privacy-enhancing proxies like Tor because of a perception that proxies are a
source of vandalism, spam, and abuse. Although these blocks might be effective,
collateral damage in the form of unrealized valuable contributions from
anonymity seekers is invisible. One of the largest and most important
user-generated content sites, Wikipedia, has attempted to block contributions
from Tor users since as early as 2005. We demonstrate that these blocks have
been imperfect and that thousands of attempts to edit on Wikipedia through Tor
have been successful. We draw upon several data sources and analytical
techniques to measure and describe the history of Tor editing on Wikipedia over
time and to compare contributions from Tor users to those from other groups of
Wikipedia users. Our analysis suggests that although Tor users who slip through
Wikipedia's ban contribute content that is more likely to be reverted and to
revert others, their contributions are otherwise similar in quality to those
from other unregistered participants and to the initial contributions of
registered users.
