Abstract—Anonymizing networks such as Tor allow users to access Internet services privately by using a series of routers to hide the

client’s IP address from the server. The success of such networks, however, has been limited by users employing this anonymity for

abusive purposes such as defacing popular Web sites. Web site administrators routinely rely on IP-address blocking for disabling

access to misbehaving users, but blocking IP addresses is not practical if the abuser routes through an anonymizing network. As a

result, administrators block all known exit nodes of anonymizing networks, denying anonymous access to misbehaving and behaving

users alike. To address this problem, we present Nymble, a system in which servers can “blacklist” misbehaving users, thereby

blocking users without compromising their anonymity. Our system is thus agnostic to different servers’ definitions of

misbehavior—servers can blacklist users for whatever reason, and the privacy of blacklisted users is maintained.