Are Tor's hidden services being overrun by images of child abuse? That's the conclusion of new research presented at the Chaos Computer Club Conference in Hamburg this week. Researchers examined directory requests to nearly 40,000 different hidden services on Tor, and found the pedophilia sites heavily overrepresented. While they represented only 2 percent of the available services, those sites made up more than 80 percent of the requests.

Only two percent of sites dealt in child porn, but they made up 80 percent of requests

The study focuses on hidden services, so called "onion addresses" that are accessible only from inside Tor. Those sites account for only 2 percent of the traffic moving through the network, dwarfed by the number of users seeking out publicly available sites. Still, the promise of effective invisibility has made them an increasingly controversial part of Tor, especially with the emergence of hidden service drug markets like the Silk Road. Hidden services are also one of the most technologically fertile parts of the network, acting as a crucial component of whistleblower tools like SecureDrop. In October, Facebook launched its own hidden service that works as an anonymous portal to the social network.

Tor officials have also raised questions about how many human users the traffic represents. Because the study measured directory requests, the data could have been skewed by unreliable sites or obsessive users that check a vast number of sites in a single session. Child pornography is also a common target for law enforcement, so it's possible some of the traffic measured in the study was the result of police officers or bots keeping an eye on the sites as part of a larger operation.