  Tor is one of the most well-known networks that protects the identity of both
content providers and their clients against any tracking or tracing on the
Internet. So far, most research attention has been focused on investigating the
security and privacy concerns of Tor and characterizing the topic or hyperlink
structure of its hidden services. However, there is still lack of knowledge
about the information leakage attributed to the linking from Tor hidden
services to the surface Web. This work addresses this gap by presenting a broad
evaluation of the network of referencing from Tor to surface Web and
investigates to what extent Tor hidden services are vulnerable against this
type of information leakage. The analyses also consider how linking to surface
websites can change the overall hyperlink structure of Tor hidden services.
They also provide reports regarding the type of information and services
provided by Tor domains. Results recover the dark-to-surface network as a
single massive connected component where over 90% of Tor hidden services have
at least one link to the surface world despite their interest in being isolated
from surface Web tracking. We identify that Tor directories have closest
proximity to all other Web resources and significantly contribute to both
communication and information dissemination through the network which
emphasizes on the main application of Tor as information provider to the
public. Our study is the product of crawling near 2 million pages from 23,145
onion seed addresses, over a three-month period.
