Anonymous communications networks were created to protect the privacy of communications, preventing censorship and traffic analysis. The most famous anonymous communication network is Tor. This anonymous communication network provides some interesting features. Among them, we can mention that Tor can hide a user’s IP address when accessing to a service such as the Web, and it also supports Tor hidden services (THS) (now named onion services) as a mechanism to conceal the server’s IP address, used mainly to provide anonymity to websites. THS is an important research field in Tor. However, there is a lack of reviews that sum up the main findings and research challenges. In this article, we present a systematic literature review that aims to offer a comprehensive overview of the research made on THS by presenting the state-of-the-art and the different research challenges to be addressed. This review has been developed from a selection of 57 articles and presents main findings and advances regarding Tor hidden services, limitations found, and future issues to be investigated.

Anonymous communications networks were born to protect the privacy of our communications, preventing censorship and traffic analysis. The most famous anonymous communication network is Tor. This anonymous communication network provides some interesting features, among them, we can mention user’s IP location or Tor Hidden Services (THS) as a mechanism to conceal the location of servers, mainly, web servers. THS is an important research field in Tor. However, there is a lack of reviews that sump up main findings and research challenges. In this article we present a systematic literature review that aims to offer a comprehensive view on the research made on Tor Hidden services presenting the state of the art and the different research challenges to be addressed. This review has been developed from a selection of 57 articles and present main findings and advances regarding Tor Hidden Services, limitations found, and future issues to be investigated.