Hidden services are websites that can only be accessed from within the Tor network, which is specifically designed to hide the IP address of both servers and users. The built-in anonymity safeguards have made Tor Hidden Services the preferred method for running online marketplaces that allow buying and selling illegal goods like drugs, guns, stolen credit card details and more.
The largest of these so-called dark markets was Silk Road, which was eventually shut down by the FBI in 2013. Many similar websites have appeared since then and some were targeted in subsequent international law enforcement raids, but Agora survived and surpassed even Silk Road in size and popularity.
In a message posted Tuesday on Agora and signed with a PGP key used for official communications, the website's administrators announced that they're taking the market offline. The stated reason was recent research about vulnerabilities in the Tor Hidden Services protocol that could be exploited to de-anonymize server locations.
The Agora administrators are working on a solution that will require "big changes" to the site's software stack, and it will take time to deploy, according to the message.
"Additionally, we have recently been discovering suspicious activity around our servers which led us to believe that some of the attacks described in the research could be going on and we decided to move servers once again, however, this is only a temporary solution," they wrote.
It's not clear which specific research the Agora administrators referred to, but a paper presented at the 24th USENIX Security Symposium two weeks ago seems to fit their description. The paper, authored by researchers from the Massachusetts Institute of Technology (MIT) and the Qatar Computing Research Institute (QCRI), describes a new traffic fingerprinting method that improves upon previous techniques and could allow attackers to determine with a high degree of accuracy where Hidden Services are hosted.
"Most of the new and previously known [de-anonymization] methods do require substantial resources to be executed, but the new research shows that the amount of resources could be much lower than expected, and in our case we do believe we have interested parties who possess such resources," the Agora administrators said.
Those interested parties are likely law enforcement agencies.
Judging by comments on Reddit, the decision to temporarily take the website offline until a solution is implemented, was largely appreciated by Agora users, especially since it's in line with the market's primary rule: "Anonymity is sacrosanct here."
The MIT and QCRI research paper proposes countermeasures that could be built into Tor itself. When reports about the research first appeared in July, the Tor Project said that it will analyze the proposed changes to the protocol to determine if they would indeed make hidden service fingerprinting harder.