Little is known about how U.S. and European law enforcement shut down more than 400 websites, including Silk Road 2.0, which used technology that hides their true IP addresses.
The websites were set up using a special feature of the Tor network, which is designed to mask people's Internet use using special software that routes encrypted browsing traffic through a network of worldwide servers.
Tor -- short for The Onion Router -- also allows people to host "hidden" websites with a special ".onion" URL, which are difficult to trace. But law enforcement appears to have figured out a method to find out where sites are hosted, a serious flaw that could pose risks to people who aren't running websites that sell drugs and weapons.
On Friday, the Department of Justice said it had shut down more than 410 hidden websites as part of "Operation Onymous" and arrested more than 17 people, including 26-year-old Blake Benthall, who is accused of running the underground marketplace Silk Road 2.0.
Hidden Tor websites have plenty of legitimate uses, and the Tor software for browsing is widely use by activists and others who want to protect their privacy.
But the use of hidden websites for selling drugs, weapons and fake passports has caused increasing concern since the first version of Silk Road was shut down in October 2013. And it appears law enforcement isn't eager to give away how it took down the hidden websites en masse.
The criminal complaint filed against Benthall doesn't reveal much other than saying that in May the FBI "identified a server located in a foreign country that was believed to be hosting the Silk Road 2.0."
The Silk Road 2.0 fell offline for some time as law enforcement officials in the country where it was located imaged the server and "conducted a forensic analysis of it," according to an affidavit included in Benthall's complaint written by FBI Special Agent Vincent D. D'Agostino.
Tor's hidden services have seen increasing use. Facebook recently announced it set up a ".onion" address to resolve performance issues some people experienced when using Tor with the social networking site.
Tor's development is overseen by The Tor Project, a nonprofit that relies in part on donations. The project "currently doesn't have funding for improving the security of hidden services," wrote Andrew Lewman, the project's executive director, in a blog post on Sunday.
"In a way, it's even surprising that hidden services have survived so far," he wrote. "The attention they have received is minimal compared to their social value and compared to the size and determination of their adversaries."
There have been various attacks developed by researchers over the years against Tor that could compromise someone's privacy, Lewman wrote. It is possible that a remote-code execution vulnerability has been found in Tor's software, or that the individual sites had flaws such as SQL injection vulnerabilities.
But Lewman wrote The Tor Project had little information on the methods used by law enforcement in the latest action.
"Tor is most interested in understanding how these services were located and if this indicates a security weakness in Tor hidden services that could be exploited by criminals or secret police repressing dissents," he wrote.
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